At the beach, a lifeguard keeps watch while dozens of people splash around in the ocean. His gaze catches something strange in the water. Quickly looking through his binoculars, he spots what looks like a shark fin approaching the shore. He runs towards the water, blowing his whistle and yelling for people to get out of the water. People scream and frantically rush out, not looking behind them. When the lifeguard walks up to the fin, which has now washed ashore, he sees that it's not a shark as he originally thought, but a woman dressed as a mermaid. The group gathers around, staring at the beautiful, and very much dead, young woman.
Callie Cargill touches up her makeup in her bathroom mirror when Detective Jim Longworth, post-shower with a towel wrapped around his waist, walks up and playfully tries to grab her. Laughing, she pushes him away, telling him that she's late for an appointment at the bank and doesn't want her to mess up her hair and makeup. She says after not getting the job at the hospital, she decided to go back to take out a loan and go back to med school. Like clockwork, Jim's cell phone rings.
At the beach, Jim and Medical Examiner Carlos Sanchez walk up to the dead mermaid. Carlos suggests that she might have been one of the performers from the local mermaid show, Weeki Wachee, which has been a tradition in the area since the 1940s. Carlos examines the detailed fin, and says that with the level of detail, it's probably custom made. He suggest that the victim's death could have been an accidental drowning, but Jim assures Carlos that if someone is going to wear a detailed and expensive-looking fin like the one the victim has on, that person probably knows how to use it. They discover she also has some broken fingernails, indicating that she either fought with someone (perhaps someone who was holding her down) or hit something with her hand once she was in the water. Jim kneels down and moves aside the victim's hair to reveal a large, hand-shaped bruise around her neck.
Jim finishes up questioning a few bystanders and walks back over to Carlos who figures she's probably been in the water for about six hours. Jim calls lab tech Daniel Green to check the tides and currents from last night when a phone rings nearby. Jim and Carlos turn to see Daniel, just a few feet away, staring at and completely enchanted by the mermaid. He eventually snaps back to reality and tells Jim and Carlos that he's been to all the big mermaid shows, Mer-Maids, the Aqua Dolls, and Chicks of the Sea, among others. Daniel tells the others he knows where the mermaid tail came from. He hands Jim an invoice for the mermaid tail, which, as Carlos suspected, was custom made. According to the invoice, the fin was $10,000 and was purchased by Cassandra Hudson. It was delivered to her apartment.
Daniel and Jim thumb through a pile of mail at Cassandra's apartment and notice that Cassandra gets the bills while her younger sister and roommate, Brooke, gets catalogues. Jim finds it a bit odd that the electric bill is so low, considering it's Florida and she should be using the AC often. Daniel speculates that maybe Cassandra didn't spend much time at home. Through the number of framed awards, certificates, and photos on the wall, it does seem that Cassandra kept busy with volunteering, teaching, and her work as a member of Sirens of the Sea, though Daniel notes that isn't one of the big shows - he hadn't heard of them before. Jim and Daniel study the pictures on the wall, noticing the same guy in a few of them, including one in of him with the Tampa Tech Synchronized Swim Team and another of him with the Sirens of the Sea. Daniel finds a Sirens of the Sea calendar, where the mysterious man is posing with the Sirens. Daniel finds a Sirens calendar. Luckily Cassandra kept note of all her appointments and they see that that the Sirens should be practicing now.
Eight pairs of long legs stick out from the pool. The legs twist and twirl, bend, and kick in perfect unison with the precision of an upside down Rockettes routine. Jay Nelson, the guy in Cassandra's photos, holds a microphone and paces on the deck, barking orders at the synchronized swimmers. Jay's tirade is interrupted by the sound of applause. Jay turns and sees Jim clapping; grinning like a schoolboy, delighted by it all. Jim flashes his badge and Jay quickly loses the attitude. Jim questions Jay, asking him where he was last night. Jay, somewhat nervous, tells Jim he was at home asleep and before that at the home of Daryl McMillian, a one-hit-wonder artist famous for the song "Boom, Bam, Bounce." Jay says that Daryl is in charge of putting the finances together for the Sirens' show, which, as Jay, the Sirens' manager, describes, will be like Cirque du Soleil in water. Daryl's throwing parties all that week for potential investors. After Jim tells Jay that he knows one of his swimmers is missing, Jay responds that he's actually missing two swimmers from practice: Cassandra and her sister Brooke. Surprised to hear that Brooke is also on the team and also not at practice, Jim tells Jay that Cassandra was killed. Jay's completely shocked and attempts to process this information. He asks about Brooke and whether or not she's a suspect before catching himself, realizing that he may have said too much. Jim says, "She is now."
At the police station, Jim tells Colleen Manus, Regional Director of the FDLE, that Cassandra's sister Brooke is apparently missing and is now a possible suspect. Manus says she'll put out a BOLO. When she inquires about whether Cassandra's family has been notified of her death, Jim says that they hadn't and Brooke was her closest next-of-kin. Manus says that now they need to be notified and they need a squad car outside their door in case Brooke shows up. Jim spots Callie sitting in his office. He walks into his office and Callie, visibly upset, tells him that she can't get a loan because she doesn't currently have a job. Rambling, Callie tells Jim that maybe she just needs to take a break and take care of herself. He agrees and goes in for a kiss when Daniel enters his office and tells him that Brooke is actually at the police station in the drunk tank, the DUI-holding cell.
A cell door slides open and a hungover Brooke steps out. Jim watches as Brooke signs out and collect her valuables. She says she can't talk about her sister right now, she's had a rough night and just wants to go for a swim, which is what she and Cassandra would always do when they needed to feel better. Jim starts questioning her, asking her where she was the night before. She says she doesn't remember. Jim asks about the big bruises on her arm, asking if she got them from fighting with Cassandra. Brooke claims she got them from the cops who brought her in. After Jim asks her again where she was that night, Brooke says she was at Daryl's party. He wanted to show off the costumes that were going to be in the show, including Cassandra's $20,000 mermaid tail (she corrects Jim when he originally says it was $10,000). Darryl insisted that the Sirens stick around and flirt with potential investors. She continues, telling Jim that she did take Cassandra's car, but doesn't remember anything after leaving the party, including where the car is. All she knows is that when she left the party, Brooke was still there. Jim, not buying her blackout story, tells Brooke not to go missing like Cassandra's car.
Jim walks into the autopsy room to meet Carlos and Daniel, but instead of Cassandra's body on the autopsy table he finds an aquarium with a small octopus inside it. Daniel tells Jim that Carlos found the octopus inside of Cassandra's mermaid tail. Carlos says one of the octopus's legs was injected with a color tattoo tag, probably as part of some kind of scientific experiment and that it's likely it is being tracked. Daniel says that with that information, they can find out exactly where this one has been and figure out exactly where Cassandra's body was dumped. Carlos then explains that, after doing an autopsy, he discovered that Cassandra wasn't drowned, but rather killed elsewhere and had her body dumped. According to Cassandra's blood composition and stomach contents, the autopsy showed that she was killed in a pool. Jim guesses that whoever drowned Cassandra had access to a pool, a car, and a lot of alcohol: like her sister, Brooke.
In the conference room, Manus briefs Jim about Brooke and Cassandra's past. She says Brooke lied about getting the bruises on her arms from the cops. They found her passed out on the street, definitely not fighting back. They didn't find Cassandra's car in the area where they picked up Brooke, either. Manus adds that Brooke's car was confiscated during a previous DUI. She's had multiple citings for public intoxication and three years in juvi under her belt. Jim and Manus compare the two sisters' phone records and financials. It's a tale of two sisters: Cassandra had money put away, Brooke didn't. Cassandra had perfect credit, Brooke's credit was non-existent. Cassandra called her parents every day, Brooke hardly ever, except, Jim points out, the night that Cassandra died. That night Brooke first called her parents, and then called her sister five times in five minutes. He reads on, and sees that both Cassandra and Brooke had a handful of late night conversations with Daryl. It sounds like the sisters were both interested in the same man.
Jim meets with music impresario Daryl McMillian, an A-list wannabe who's dressed in a silk robe, at his mansion. They head into the kitchen where Daryl tells Jim about his big plans for Sirens of the Sea. Jim takes a peek at some documents on the counter, but Daryl quickly takes them away. Jim spots a beautiful pool and huge boat, a 40 foot cruiser, in the backyard. Daryl says the boat isn't working, the engine is in a dry dock, missing a part. When Jim implies that Daryl and Cassandra were an item, Daryl dismisses that idea, saying their relationship was strictly business. But he says everyone knows that Jay Nelson had been carrying a torch for Cassandra since college. As Cassandra's death finally hits Daryl, Jim notices an envelope from First Trust Bank addressed to the Sirens. Daryl tells Jim that Cassandra was the total package - beautiful and talented. It was her idea to take the Sirens to the next level by doing the big show. He says her sister Brooke was a hot mess, and that the night of the party, after she got drunk she took Cassandra's car and left Cassandra stranded at the party. Everyone knew Cassandra's whole life was in her car. Jay Nelson gave her a ride home that night.
Callie sits in front of the TV, barely watching, when her son Jeff rushes through the front door and into the kitchen. Callie quickly joins Jeff in the kitchen and attempts to make small talk. He acknowledges his mom by giving her one-word answers to all of her questions about school and his friends, then turns to head up to his room. Callie stops him and says that she just wants to enjoy spending some time with her son and to remember that they're playing Yahtzee tonight. Jeff turns to head upstairs to his room. Callie watches him, realizing that she's losing it.
At the lab, Daniel tells Jim that Tampa Tech is sending over Jay's files. Jim also requests that Daniel get whatever he can on Daryl's finances. When Carlos comes over, Daniel tells them that he sent the octopus to the Marine Life Institute to get checked out, and he was able to figure out where Cassandra's body was dumped from: the 14th Street Causeway. Thirteen juvenile octopuses were living there. Based on tides and currents, Daniel was able to determine the body was dumped at a nearby location and then passed by the Causeway before the body was pulled into the ocean. Carlos adds that there's a road that leads right to the area next to the Causeway and it's only a short drive from there to Daryl's house. It's all adding up. Cassandra drowned in Daryl's pool, someone put her in the mermaid's tail that was on display in Daryl's house to make it look like her drowning was an accident and someone drove to the bay to dump her body. Carlos theorizes that instead of being brought to the bay by car, it could have been brought by boat. Daniel says that he confirmed Daryl's cruiser engine was in dry dock. Carlos says maybe Daryl just rolled the body right off his deck, but Daniel says that if that was the case, the body would have taken a couple of weeks to wind up in the ocean instead of just six hours. Plus, Jim adds, Daryl doesn't have a motive because he's the one person who needed Cassandra to stay alive.
A party is in full swing at Daryl's house and the Sirens are performing in the water. Jim joins Carlos, handing him a beer while surveying the scene. Jim tells Carlos that he spoke to someone at the catering company and was told that at the last party Cassandra and Brooke had a fight. He spots Brooke, cozying up with a potential investor and Jay, who's documenting the party with a video camera. He approaches Jay, and tells him that he knows he gave Cassandra a ride home the night she died. Jay denies driving her home and killing her. Jim hands Jay a search warrant and, reluctantly, Jay hands over his car keys.
In the interrogation room at the station, Jim tells Jay that they found scales from Cassandra's mermaid tail in his car. Jay says that he brought the costume over to Daryl's house because Cassandra had too much stuff in her car to do so. Jim says that the techs found a dried, foamy fluid in the car, which is consistent with the liquid that comes out of a person's lungs after they have drowned. Jay maintains that he and Cassandra were friends and he didn't kill her. But Jim says everyone knows he wanted to be more than friends with Cassandra, ever since college. Jim whips out Jay's files from Tampa Tech. Sullen, Jay already knows what Jim is going to bring up - a hit and run accident he was involved in during college that totaled three cars. Even though he was acquitted in his trial, he was kicked out of the water polo team and suspended from school. Jim tells Jay that since Jay's infatuation and loyalty to Cassandra ran deep, it was clear that he took the blame for the accident even though Cassandra was driving. Jay admits to taking the blame, only because Cassandra would have lost her swimming scholarship if he didn't. Jim doesn't buy his story and tells Jay that Cassandra was ignoring him after everything he did for her must have upset him. Jim knows Jay didn't drive Cassandra home that night. She got out of the car and went back to Daryl's house. He then followed Cassandra back to the house and killed her in the pool. Jay denies it entirely, but Jim, not listening at all, gets up and heads to the door.
Back at Daryl's house, Jim tells Daryl that the night Cassandra died, she left with Jay, but came back to the house to be with him. Daryl tells Jim that Cassandra was not his type, she was too much of a goody-goody, and that if she did come back to the house, it was probably to use his pool, which she was welcome to do whenever she wanted. He didn't hear her because the house is so large. Jim asks if he can look around and Daryl agrees. Jim then gestures out the window to a handful of FDLE uniforms searching the grounds and says they'll also going to have to take the small skiff off his dry dock cruiser as evidence.
In her kitchen, a nervous Callie is on the phone, asking to speak to Dr. Avery about a job opening at Atlanta General Hospital. Just then, the doorbell rings and Callie quickly hangs up the phone. Jim walks in, takeout food in hand. They chow down while Jim runs through the case with her, saying that he doesn't understand why someone would go through the trouble of putting the fin on Cassandra before dumping her in the ocean. Callie suggests that maybe Cassandra put the beautiful fin on herself to take for a test swim. Jim considers this theory.In the autopsy room, Carlos tells Jim that the fluid found in Jay's car was actually vomit, which most likely came from Brooke , as the DNA wasn't Cassandra's, but was from a close relative. Carlos said they haven't had any luck finding Brook's car, but Daniel was able to retrieve the voicemails Brooke left for Cassandra on the night she was killed. Daniel plays Brooke's voicemails for the group. In the message, Brooke says that she knows all about Cassandra and Daryl and that she's so angry at Cassandra that she could kill her. Brooke looks like Jim's prime suspect now.
Jim enters Daryl's dimly lit deserted backyard. He sees someone at the bottom of the pool. It's Brooke. Jim jumps in, swims to the bottom, and grabs her. She gasps for air and pushes him away, telling him that she was practicing holding her breath since she's the new lead in the Sirens show because "the show must go on." After Jim tells Brooke she was jealous of Cassandra, she says that the idea for the show was her idea. She brought the idea to Daryl months ago, but when he met Cassandra, he forgot that Brooke existed. But she was resigned to being the "bad sister." Cassandra had it all: looks, talent, men. She complains that good guys like Jay never fall in love with her, but she's not bad. When Jim asks about the car, she maintains that she doesn't know where it is. Rolling her eyes, she tells Jim she has no recollection of anything except drinking a lot of vodka and "flaming shots" of tequila and rum. Seeing that his phone is waterlogged, he asks to use Brooke's phone, knowing exactly who to call.
Jim and Carlos pull up to The Firehouse, home of Florida's famous flaming shot. They pull next to a Cassandra's abandoned car and find it packed with stuff. It looks like she was living out of it.
Back in the evidence room, Jim and Carlos dump all of the contents found in Cassandra's car on the table, including a suitcase filled with winter clothes and a laptop. Carlos notes that it looked like Cassandra was looking to relocate, not go on vacation. Jim picks up a large envelope after noticing it said Atlantic Commerce Financial on it, which was also the name of the bank statements that Jim glanced at when he was at Daryl's house. Carlos notes that it's for the Sirens of the Sea, but is addressed to Daryl. Jim said that so was the statement he saw at Daryl's but that this envelope was mailed from North Carolina, whereas the statement at Daryl's was postmarked from Palm Glade. Jim notes North Carolina is where the bank is actually located. Carlos wonders aloud what Cassandra is doing with Daryl's bank statements. Jim doesn't know, but says that Daniel looked into everyone's personal finances and didn't find any impropriety. Daniel then hands Jim his new cell phone, and says he downloaded some promotional Siren footage off of Cassandra's computer onto it that he thinks Jim should look at. Jim plays the footage. It's Jay filming the Sirens at practice. But then Cassandra appears and starts screaming at him, saying that the investors want to see the star (her) not the chorus. After seeing Cassandra's bad attitude, Carlos has his doubts that she was the "good" sister. Jim theorizes that Jay probably holds a torch for Brooke, not Cassandra.
Back at Daryl's mansion, Jim finds Jay walking along the dock towards Daryl's boat. Jim stops him and says the skiff is no longer there, that it's being held as evidence. Jay denies looking for the boat. He says the Sirens have a show there that night. Jim shows him more of the footage on his phone and points out that it's focused on Brooke, who wasn't the star of the show four weeks ago. He says he thinks Jay's in love with Brooke, not Cassandra and that he can't cover up for Brooke anymore. Things have gone too far. Jay said the night he was driving Cassandra home from Daryl's party, he told her that he didn't like the way she was treating Brooke. They got into an argument and that's when she got out of his car. Jim tells him that Jay looks guilty. Jay tells Jim that if he's there to arrest him to do it already. But Jim is waiting on evidence and isn't ready to arrest anybody yet.
Jim and Callie dance at a thatched roof shack on the beach, overlooking the water. The floor is packed with couples dancing. After the songs "Boom, Bam, Bounce" comes on, the two laugh and go to sit down. Jim tells Callie that he wants to co-sign the loan for her so she can go back to school. Although appreciative, Callie assures Jim that she'll be fine and has decided to pursue a career in a different field, at least for now. She's open to do anything health-related, even if it's at a spa or yoga studio. Carlos calls Jim and tells him Cassandra's DNA was found on Daryl's skiff.
Daryl's backyard is dramatically lit, a spectacular setting for the Sirens final investor performance. The lights go down and the Sirens, all matched in beautiful sequined suits, dive into the water. High in the air, Brooke sits atop a circular swing and descends towards the pool. Jay spots Jim and asks what he's doing there, that he told him earlier he wasn't ready to make any arrests. He says Jay's only guilty of protecting Brooke, and beckons to Brooke, telling her that "the show's over."
Jim stands at the pool with Brooke, now wearing a robe over her bikini. Jay is by her side. He tells them that Cassandra stole $5 million from investors, including $40,000 from her parents, for the show. Brooke tells Jim that she didn't want Cassandra screwing over their parents, especially since they invested their life savings into the Sirens. Jim says that Brooke must have found out about her sister's plan and wanted to stop her. Jim mentions the one-way ticket to Switzerland Cassandra bought for herself. This catches Brooke and Jay completely off guard and Jim explains that Cassandra never planned for the show to actually open, she just wanted to churn investors. Brooke is shocked to hear this and Jay is shocked that Brooke really didn't kill Cassandra as he originally thought. Jim figures out that neither of them had anything to do with the murder.
In the interrogation room, Daryl questions why he would kill Cassandra if she was the star of his show. Jim says they were churning investors together. She was cooking the books. They were claiming Cassandra's new mermaid tail cost $20,000 when the person who made it only charged them $10,000. Daryl denies any involvement. Accounting was Cassandra's department and he trusted her. He finally admits that the books were off, but he wasn't going to kill anyone over $10,000. But Jim says the amount was actually $5 million. Cassandra created fake bank statements and snail mailed them to Daryl. The real ones were in her car. That's why Daryl didn't know that she had actually withdrawn all that money and transferred it into a Sirens account that only she had access to. Jim says Daryl only realized that he had been played the night Cassandra was murdered, when he tried accessing the account online three times but was denied. Cassandra had changed the login information. Jim says Cassandra did get a ride with Jay, but that when they got into an argument, she left him and went back to Daryl's house. She couldn't resist trying on the tail and taking it for a swim. That's when Daryl killed her. With his cruiser out of commission Darryl had to use the skiff to dump Cassandra's body, where the lab techs found scales from the mermaid tail and dried, aspirated foam that matched Cassandra's DNA. Daryl glowers. He can't argue with the cold hard facts.
Jay and Brooke sit on the sand, staring out into the ocean, relieved that everything is finally over. Brooke says that she was hoping to be as good as Cassandra in the show so that she could help her parents get their money back and maybe everyone would look at her differently. Jim says that Cassandra was looking to screw everyone over except for her parents. When she transferred the money out of the Sirens' account, she transferred the $40,000 back into her parents' account with 20 percent interest. The money already cleared, and because Cassandra took that money and put it into another corporate account for the Sirens and not a personal one for herself, the courts probably won't freeze those assets as part of the criminal case against Daryl. The show can still go on. Brooke and Jay thank Jim for figuring everything out. He smiles, telling them it was easy once he realized they were in love.
Rushing out of the house, Callie opens her door to find Jim. She says she's on her way to take a class at the community college that could help her get a job with an acupuncturist. Jims stops her and says that she's making all of these sacrifices to be with him and that she doesn't have to do it. He tells her that she should do what she was meant to do and take the job in Atlanta. He assures her that they can make it work and that Atlanta is only an hour-long plane ride away. Callie looks at him, knowing he's right. She laughs and smiles through her tears as Jim gathers her up into his arms. She's going to Atlanta for the job.