A woman's body is discovered tumbling around inside a clothes dryer in the basement of an Upper West Side apartment building.
At the precinct, Castle is lectured by a lawyer about signing papers that waive all his legal rights in exchange for the opportunity to accompany Beckett on her rounds. As Castle kvetches, Beckett gets a call -- it's a new case. Castle hurries to sign the paper as Beckett leaves without him.
Beckett arrives at the 86th Street crime scene to find Ryan and Esposito in the building's basement laundry room. Apparently an elderly female tenant opened a dryer and discovered a young woman's body inside. The girl was a nanny and worked in Apartment 12F.
Beckett finds Castle waiting for her on the 12th floor, having been told about the case by Captain Montgomery. He gives her a quick background on the dead nanny -- her name was Sara Manning and she had been working for the Peterson family for about two years. "The doorman's a huge fan of my work," he explains.
Castle and Beckett interview the nanny's former employers, Claudia and Howard Peterson. Sara had been taking care of their 8-year-old son, Justin. After Sara failed to pick up Justin from school, Claudia tried to call her, but got no answer so she took a cab to get her son. When they arrived home, the police were already there.
Castle asks if Sara had a boyfriend. "Brent," replies Howard, "but they broke up." Claudia doesn't have the Manning family's contact info, but she offers Sara's jacket and bag to them. In the elevator, Beckett digs through the purse and jacket, but can't find Sara's cell phone.
They return to the precinct to find Montgomery, Esposito, and Ryan studying the video in which Sara is seen entering with a laundry basket and exiting at the basement. A few minutes later, she gets back upstairs. Forty minutes after, she enters the basement for a second time and is never seen again. The only other person seen riding the elevator is the elderly woman that discovered Sara's body. The cops conclude that the killer must have used the stairwell or already been in the basement when Sara arrived.
As Ryan and Esposito run social security numbers on all the building's workers, Castle wonders why they don't run the numbers for the neighbors. He then proceeds to spin a gripping tale about the potential for a stalker neighbor. The cops hang on his every word and afterwards, Montgomery decides they should run numbers on all the tenants, too.
At the morgue, Lanie Parish explains that Sara was killed by a blow to the back of the head, and then struck her temple on the edge of the folding table. They also learn that Sara had sex shortly before her murder and the man wore a condom.
At the precinct, the cops are skeptical of the sexual predator theory -- rapists don't ordinarily wear condoms. There's also still no word on the missing cell phone. Esposito, however, has gotten the service provider to "ping" the phone. It's near or inside the apartment building.
Looking at the phone records, Beckett notices dozens of calls to Sara from the same number in the past two months. She also sees that Sara stopped returning the calls a week before they stopped. The calls were from Brent Johnson, her ex-boyfriend.
Castle and Beckett interrogate Brent, who admits that Sara and he split but claims it was mutual. When he's confronted with the threatening voicemails he left, Brent finally admits that he was dumped. However, he has an alibi for her murder -- he was at work. He found out about Sara's death through a mutual friend from college, Chloe Richardson, who nannies in the building for another family and had referred Sara to the Petersons.
As the two approach a playground in Central Park, Castle reminisces about the times he spent there with Alexis when she was little, which surprises Beckett as she never figured Castle for "Mr. Mom."
They spot Chloe with the little girl she's babysitting, Becca. Beckett tells Chloe that Brent thought Sara was seeing someone. Chloe eventually reveals that Sara had started staying late for the Petersons -- but only on the nights Howard Peterson was home.
Afterwards, Castle chides Beckett for not asking Howard Peterson more questions when they first spoke to him. But Beckett says she always waits until she has more information before she interviews her primary suspect. This way, the alleged perp is tied to more specific answers.
Beckett confronts Howard with the information from Chloe. At first he denies it, but after some prodding from Castle, Howard admits to having an affair. Except it wasn't with Sara. In fact, Sara would stay late to cover for him; he would call the apartment from his cell phone to make sure everything was okay. They can verify it all with his phone records. Esposito then tells Becket that Sara's ex-boyfriend's alibi checks out. Now all they have is a cheating husband with a possible alibi. That's when Ryan jumps in: "Guess who wasn't where they said they were the day Sara Manning was killed?" The answer, it turns out, is Claudia Peterson.
Castle and Beckett question Claudia. The day her son's school tried to reach Claudia at her office, her assistant said she wasn't there. Claudia confesses she was actually at her lawyer's office, getting ready to serve her cheating husband with divorce papers.
At home, Castle studies the elevator camera videos. Sara Manning appears on two monitors -- riding to the basement with the laundry basket and, the second time, riding down to put the clothes in the dryer. He notices there's a 5 second difference in the time stamp for the two trips. Why would it take her 5 more seconds to travel the same distance? Castle and Beckett then ride the elevator with Castle timing their ride from the basement. When the doors open, they're on the 15th floor. That's where Sara was coming from when she went down to the basement to put the clothes in the dryer. But that makes no sense. The Petersons live on 12.
Surmising that whoever wore the condom must live on the 15th floor, Castle starts knocking on doors. An elderly man answers, but Castle's looking for someone younger. Just then, a door opens down the hallway and a woman, Diana Harris, exits an apartment with Becca, the little girl in the park whom Chloe Richardson baby-sits.
As Beckett and Castle question Diana, her husband Ian saunters in from the bedroom. Diana suddenly remembers that Ian was home the day of Sara's murder. Castle and Beckett both notice Ian's anxiety. Castle then asks to use their bathroom.
In the bathroom, Castle pokes through the Harris's medicine cabinet and finds a box of condoms. Ian Harris tells Castle that he was sleeping in the bedroom, so he can't say for sure if Sara stopped by. That's when Castle surreptitiously dials a number and a phone begins to ring somewhere in the apartment. Diana, now noticing Ian's anxiety, gets up to find Sara's phone in the bedroom.
Back at the precinct, Ian Harris confesses to having sex with Sara, but he swears he never hurt her. He tells them that Sara left his apartment about 10 minutes before Chloe got back from the park.
They review the elevator footage and see Sara getting on at 12:48. Then they see Chloe getting on at 12:54. So it looks like Ian Harris was telling the truth: There's no way he could have gotten down to the basement and back upstairs in time to see Chloe. That's when Beckett notices Becca isn't with Chloe in the elevator tape. As Castle notes, coincidences don't just happen -- not when a murder is involved. Chloe would have known from Sara's schedule that she'd be in the basement.
Beckett and Castle visit Chloe's apartment. Her roommate Bethany claims Chloe had to go baby-sit. Castle then notices a photo in Chloe's room -- Ian and Becca Harris with Chloe -- Diana Harris having been cropped out of the photo. Clearly, Chloe is in love with Ian.
Castle and Beckett arrive at the apartment building and find Esposito and Ryan waiting. The doorman sure both Chloe and Ian Harris are upstairs, though no one's answering. Upstairs, they find Ian Harris passed out on his living room floor and Diana and Becca locked in the bedroom, safe. The doorman then calls -- a tenant saw a girl in the laundry room... and she has a knife.
In the laundry room, Beckett finds Chloe sitting in front of a dryer, holding a butcher knife. Chloe tells them she and Ian had been having an affair... and that she's pregnant with his baby. When she found out that Sara was sleeping with Ian, Chloe killed her in a jealous rage. Explaining that she can empathize with Chloe's heartbreak, Beckett's able to convince Chloe to put the knife away.
Later, Castle compliments Beckett on the sisterhood act she ran with Chloe, but Beckett says that it wasn't an act. What Ian Harris did has consequences -- and, unfortunately, he'll be able to just walk away. Castle doesn't think so. He sees a big, messy divorce in Ian's future.
At home, Castle writes on his laptop about discovering Nikki Heat's heart was broken -- clearly inspired by what Beckett said. Alexis stops by to ask if they caught the culprit and if Castle was right about who the killer was. In fact, Castle tells her, he wasn't. Alexis chides Castle and reminds him that it's fun to be surprised sometimes. As she exits, she thanks him for having been her nanny. "No sweat, kiddo."
Castle pauses in his writing and reaches into a desk drawer. He pulls out a cherished photo of 4-year-old Alexis strolling in the park, hand in hand with him, and smiles at it.