A late-night movie-going couple, Patty and Glenn, walk across a parking garage. As Patty and Glenn reach the stairwell, their car alarm goes off behind them. They turn to see a trench-coated figure trying to open their car. Glenn yells at him to get away. When the "thief" turns, Glenn and Patty see that he's a disheveled businessman-type - with a knife sticking out of his bloody crotch. Glenn yells for Patty to call 911, then grabs the knife handle and pulls it out. Blood spurts from the wound.
Stabler and Benson walk with Dr. Manning. They discuss the fact that the knife is contaminated evidence, because Glenn yanked it out, then when the artery spurted, Patty tried to stick it back in. Besides his crotch, the victim was also stabbed in his abdomen and has a defensive cut on his right hand. The victim's high B.A.C. (blood alcohol content) of 1.9 likely blunted the pain. The victim lies unconscious in the ICU. His biggest problem is bleeding in the brain caused by banging his head. He's comatose with no ID whatsoever, thus a John Doe. Stabler comments that they'll try to get an ID by fingerprinting him and getting a DNA swab. Benson and Stabler consider the crime - it could be a sex crime, revenge attack, a botched robbery or all of the above.
Stabler interviews the parking garage attendant, Malik, as they look at security footage of John Doe walking out of, then into the parking garage. Malik reports that a drunk John Doe banged on his window, wanting help finding his car, but Malik's boss doesn't like him leaving the booth. Malik claims that he never saw the knife because the counter was in the way. Benson approaches, shares that she found some bloody shoe prints and sent Patrol to track them. Just then, Fin radios for them to come to Level Two.
Fin shows Benson and Stabler a car that is the same make and model as Glenn and Patty's. The registration yields the name "Jerry Bullard" and a photo ID badge indicates that he works at Bullard Aviation Systems (B.A.S.). A sheaf of photos in the trunk reflects that B.A.S. makes aerial drone weapons for use in Afghanistan. Fin comments that Bullard kills for a living.
Benson and Stabler go to the Bullard home to question Jerry's wife, Jane. Jane clarifies that Jerry doesn't kill people, he designs the tires, landing gear and struts on the drones, and doesn't even have a security clearance. So the attack likely wasn't work-related. Just then, Jane's daughter, Jeanette, 16, returns from school. Jane tells her that her father is hurt and in the hospital. Jane and Jeanette are both confused, because they thought Jerry had gone to D.C. for meetings at the Pentagon. Benson says he never went; they found his suitcase and boarding pass in the car. Benson asks Jane if she and Jerry had been having any marital problems. Jane insists that Jerry wasn't cheating and is as vanilla as they come, adding that their sex life is great. Benson turns to Jeanette, who also confirms that her dad is normal and could even be described as prude.
Back at the precinct, Stabler and Benson watch a promotional video for B.A.S. featuring Jerry. Cragen enters, and they report that Jerry is squeaky clean. Wherever he went was within walking distance of the parking garage. Benson brings up a map of the area with markers showing where bloody footprints were found, which demonstrates that Jerry wandered aimlessly after he was attacked. They couldn't find a single bar that served him, and Jane said he left home sober. Fin reports that there is a suspect - a bunch of people said they saw the neighborhood weirdo wearing bloody clothes that night. The suspect is named Aldo, an Italian immigrant who lost his job in a butcher shop a year ago, after which his wife split. People in the neighborhood call him the Night Stalker because he started walking around the streets, acting strange.
Benson and Stabler stake out the area, looking for the Night Stalker. Finally Stabler sees a man who matches Aldo's description wearing a bloody shirt. He and Benson stop him, ask if they can talk. Aldo claims he didn't do anything and he just wants to sleep. Back at the precinct, Huang shows Aldo a photo of Jerry Ballard. Aldo has difficulty focusing, but he might recognize Jerry. Aldo admits that he has trouble keeping track of time and claims that it's because he's been experiencing insomnia for a year and never sleeps. The inability to focus caused the accident that cost him his fingers at the butcher shop, caused his wife to leave him and is the reason he wanders the streets at night. Aldo also sweats constantly, a symptom that started about a year ago. Huang takes a look at Aldo's eyes. His pupils are tiny.
Huang steps out to the observation room. He explains to Benson and Stabler that Aldo likely suffers from a genetic, neurodegenerative disease called Fatal Familial Insomnia. It mainly affects families from a small town in Italy like Aldo's, and the disease is usually fatal from exhaustion within 15 months of the first symptoms. Stabler comments that Aldo could still be the killer, sharing that CSU found that the blood on Aldo's shirt is the victim's. Just then, Aldo taps on the glass holding Jerry's photo. He claims he remembers where he saw Jerry on Monday night.
Huang, Benson and Stabler bring Aldo to an alley where he says Jerry ran into him, which is how Jerry's blood got on Aldo's clothes. Aldo walks them through the sequence of events. He was out walking when he heard a commotion, something that sounded like someone was being murdered. That was when Jerry came around the corner covered in blood and ran into him. But Aldo never saw Jerry's attacker. Aldo points to a nearby unmarked doorway and says that he's seen nicely dressed people going in and out when he walks by, as he did on Monday night. That's all Aldo can offer them. Huang wants to admit Aldo to the hospital to try and help him. Benson knocks on the unmarked door. There's no answer and it's locked.
Back at the precinct, Benson, Stabler and Fin are gathered in the media room. Fin shows them a website, explaining that the door is to a place called The Swing Set, which is a swinger club. It's not your father's swingers' club, though - it's sleek and modern. Club rules require new members to be invited by a current member. Benson asks Stabler how he wants to play it. Stabler says they'll tell them their good friend Jerry invited them.
Benson and Stabler enter The Swing Set, dressed for their undercover assignment. They're approached by a greeter, Suzette. Benson tells her Jerry is their referral and Suzette offers that he's one of their best members. She leads them inside.
Spider, the bartender, gets Benson and Stabler club sodas. Stabler mentions that Jerry is supposed to meet them, asks if Suzette has seen him. She hasn't but advises them to look for Cassandra if they want to find Jerry. Suzette walks over to the beautiful Cassandra and whispers in her ear. Cassandra approaches, chats with Stabler. She confirms that she knows Jerry. A young couple, Scott and Leila, approach Benson. She identifies her as a "newbie." Cassandra leads Stabler off, leaving Benson with Scott and Leila. Cassandra brings Stabler to the back corner of a "Play Room." Stabler asks her about Jerry. She recalls that they had fun Monday night, but Jerry had too much to drink and went outside for some fresh air. Cassandra lost track of him after that. When Stabler asks if Jerry left with anybody, Cassandra comments that he asks a lot of questions.
Back at the bar, Benson talks to Scott and Leila about Jerry. They've only met him a couple times, but noticed he only has eyes for Cassandra. Both Benson and Stabler ask about jealousy issues. Stabler asks Cassandra about a scar on her neck. She says that her ex, Doug, couldn't handle the "lifestyle." Doug made a big scene about a week ago when Cassandra was having fun with Jerry. Doug came barging into the Play Room, screaming about how Cassandra "belonged" to Doug. Doug tried to attack Jerry, but Jerry got in Doug's face and defended Cassandra. Scott and Leila describe how surprised everyone was, because Jerry's usually quiet, and how everyone was turned on after that, including Jane Bullard. It's news to Benson that Jerry brought his wife there.
Benson and Stabler question Jane at the hospital. Jane points to Jeanette as the reason she didn't tell them about The Swing Set. She also doesn't like being viewed as a deviant, reminding them that swinging isn't illegal. Stabler tells Jane that Jerry went to The Swing Set on Monday night instead of leaving for D.C., and was attacked outside. Jane feels betrayed, she and Jerry always promised each other they'd go together and leave together. Benson asks if Jane ever worried about Cassandra's ex, Doug. Jane doesn't know Doug's last name, but can help describe him to a sketch artist.
Benson and Stabler return to The Swing Set. Cassandra finds Stabler, asks if he's ready to do more than watch tonight. Outside, a man on a motorcycle pulls up. Fin checks the man's appearance against the sketch, then radios that Doug's on his way in. Doug storms into The Swing Set, headed right for Stabler and Cassandra. Doug gets in Stabler's face, so Stabler pins him, pulls out his gun. Fin announces them as NYPD. When Cassandra hears that, she slips away unnoticed. Stabler takes a knife from Doug's back pocket. Fin helps Benson drag Doug out and Stabler turns to get Cassandra to come to the precinct as well, but she's disappeared.
At the precinct, Stabler interrogates Doug. They verbally spar, then Stabler asks Doug about the scrape on his face. Stabler wonders if Doug got it attacking Jerry Bullard outside The Swing Set on Monday night. Doug denies stabbing anyone, claims that he got the scrape at the same time he lost his old knife - when some idiot opened his car door into traffic and Doug had to lay his bike down. Doug demands a lawyer.
In the observation room, Cragen and Hardwicke aren't happy because Doug didn't give them much. Hardwicke questions whether Doug is the one who attacked Jerry. They've got nothing to nail him, nothing to tie him to the alley or the knife and their only witness, Cassandra, bolted. Fin walks in and announces that Jerry might be able to help nail Doug - he just woke up.
Benson and Stabler visit Jerry at the hospital. His daughter is by his bedside. Jerry confesses to Benson and Stabler that he's in love with someone else, Cassandra. He stopped on his way to the airport because he had to see her. Jerry remembers who attacked him.
Benson and Stabler question Jane, accusing her of trying to kill Jerry. Jane insists that she didn't care about Jerry sleeping with Cassandra, what she cares about is Jerry destroying their future. He was pulling tens of thousands of dollars out of Jeanette's college fund and giving it to Cassandra. She confesses that she called the hotel in D.C. and learned that Jerry never checked in. Jane knew he was at The Swing Set. She waited in the alley for Jerry. Before she knew it, she was stabbing him. Jane claims she was glad to hear he was alive the next morning; she was very drunk when she stabbed him.
Cragen, Fin and Hardwicke discuss that they have to release Doug. They try to come up with charges to keep him there, but ultimately Cragen tells Fin to cut Doug loose. Cragen and Fin release Doug from the interrogation room, but not before Doug gets one last dig in, threatening Fin to stay away from Cassandra. Fin believes Doug will go after Cassandra again, commenting that it's hard to protect her if she's in the wind. Cragen has an idea - he'll put Benson and Stabler on Doug's tail to make sure he doesn't get the chance.
Doug rides his motorcycle up to an apartment building. Benson and Stabler tail him. As Doug starts to enter the building, Cassandra exits, freezing when she sees him. Benson and Stabler move to intervene, then see Doug and Cassandra embrace and kiss. Benson and Stabler realize that they've been played. They arrest Doug and Cassandra.
Stabler interrogates Cassandra. He accuses her of being a con artist who fleeces guys like Jerry Bullard out of their life savings. Cassandra plays the jealousy card and gets guys like Jerry to help her. Benson questions Doug at the same time. She tells him that they've found numerous identities for him and Cassandra. Benson and Stabler know Doug and Cassandra's game. Their victims won't go to the cops because no one wants to admit they've been a patsy and a swinger. Doug says they don't have a case. Benson is certain she can convince Jerry to testify against them. Cassandra claims Jerry loves her. Stabler reveals that when they're done with her and Doug, they'll get a free trip to Miami to face charges related to a sex club they hit the previous summer, in the death of a swinger named Claude Bailey.
Benson reminds Doug that Claude ended up face-down in a pool of blood, with his bank account drained of $50,000. They slipped up in Miami, because police found skin cells under Claude's fingernails and Stabler's certain that the DNA will match one of Cassandra's or Doug's samples. Cassandra seems like she's about to talk when an attorney walks in. Jerry sent him. Cassandra triumphantly tells Stabler that she knew Jerry loved her.
At the arraignment hearing, the judge criticizes Hardwicke for the volume of charges. Hardwicke defends the quantity by arguing that the defendants are serial con artists who've defrauded countless victims. Hardwicke identifies Jerry as one of those victims, despite the fact that he's paying for Cassandra's lawyer. Both Doug and Cassandra plead not guilty through their lawyers. Hardwicke argues that they're flight risks and shouldn't be granted bail. The judge orders them to surrender their passports, but allows bail to be set at $250,000 each. Jerry promises to post Cassandra's bail. Doug asks her about his bail, but she ignores him. Doug yells obscenities after her. Benson and Stabler warn Jerry that Cassandra is playing him, but he's too in love with her to see it.
Back at the precinct, Fin reports that Miami P.D. is faxing them the DNA results, and they're unbelievable. The skin cells under Claude's fingernails are a match with Doug. And the DNA showed one more thing...
Benson and Stabler find Jerry waiting on the street with roses. They reveal that Doug and Cassandra are brother and sister. Jerry can't believe it. He's devastated because he loves Cassandra and he's convinced they're soul mates.
Benson and Stabler wait for Cassandra outside her building. She brags that Jerry invited her to live with him. Benson says that they know she and Doug are siblings. Cassandra reveals that they're not only siblings, they're twins who share a soul. Benson reminds her it's still incest. They tell her that her gravy train with Jerry is over, but she laughs, says Jerry just told her to come home soon because he has the "perfect gift" for her.
Benson and Stabler rush to Jerry's house. They find bail papers for Doug. Jerry describes how easy it was to get Doug there, how Doug never even saw Jerry pick up his knife. Jerry reveals Doug dead on the couch with his own knife rammed in his gut. Jerry declares that now he and Cassandra can be together forever. Cassandra arrives and takes in the scene, screaming. There will be no happily ever after for any of them.