The episode picks up where last week's left off, with the discovery of the murdered Brian Goudy. Sam fills in 15 Division on the situation: Brian went missing after taking an unknown person for a test drive in his black car. The day before Brian was killed, Doctor Milo Howard reported a suspicious man who test drove his car, the same kind as Brian's. Texts confirm it's the same buyer. The running theory is that Brian's murderer is a serial predator who uses website to find victims. Howard has described the suspect as a tall male Caucasian in his mid 30s with dark hair, weighing about 195 pounds. The suspect was driving a silver sedan, for which the officers have recovered partial plates.
Gail and Nick are assigned to tracking down the sedan and Chloe and Dov are assigned to finding out more about Brian. Traci arrives at the door of the parade meeting with Brian's twin brother Jim in tow. After he sees the pictures of his brother's body on the wall, Sam and Andy console him elsewhere in the station. Jim says he and his brother were very close but he didn't know about Brian's plans to move to Costa Rica. He says Brian was so kind, he has no idea who would have wanted to hurt him.
Shoot to Traci trying to get a hold of Dex, who didn't show up to their custody mediation meeting, a good thing for Traci, though she feels guilty thinking that. But Steve reminds her that even though it's a battle she doesn't want to fight, she needs to win it. The pep talk ends when Chris approaches Traci with news that no other precincts have reported any crimes linked to online auto sites, so it's not likely that their suspect is a serial predator. This frustrates Traci since it doesn't seem like there is a reason Brian would be anyone's target.
In the midst of Chloe and Dov's search to find a reason, Dov makes a joke about people hiding things like husbands, which leads to a passive-aggressive fight between them.
While Nick and Gail are online searching for the silver sedan, Gail tells Nick she's going to make him an online dating profile, joking about all the weird things she'll say he does. He turns the tables and says he could do the same for her. Aw how cute- it's the first sincere, tension-free sign that they have become friends again after everything that happened between them.
When Traci asks them what they've found, Nick says there are eight silver sedans with the partial plates. They find a possible match in Sean Harrison. When they arrive at Sean's house, he's odd but cooperates with them.
Once he's at the station, Sam tells Sean that his car was seen at the gas station where the suspect met Doctor Howard and Brian Goudy. Sean claims he's never been to that gas station or heard of Brian. Outside the interrogation room, Doctor Howard says Sean is not the guy he met. Then Kelly, Sean's wife, arrives and when Sam asks her if anyone else has access to her husband's car she mentions a guy named Larry. Kelly explains that Larry is someone who grew up with Sean and he stays in their basement whenever he gets out of prison-- which is recently.
Sam and Andy confront Sean about keeping Larry a secret and Sean says he wanted to give Larry the benefit of the doubt, but admits he doesn't know where Larry is now and that he wasn't with him on the afternoons in question. Sean informs them that Larry had a parole meeting that morning, but he doesn't have anyone on the outside he'd go see, just people in prison who had watched his back while he was there. Doctor Howard IDs Larry as the suspect.
Traci, Oliver and Sam fill everyone at 15 in on the latest developments, giving a working theory that Larry was doing a favor for someone on the inside. Soon Andy comes into the room and informs them that Larry made his parole board meeting but was a nervous wreck, and he had indeed contacted Doctor Howard and Brian, as well as made three calls to an illegal cell phone in the prison he'd been in, the day after Goudy's death. Sam makes a call and gets a list of people in prison who Larry knew. Sam insists on being the one to go talk to them and Andy suspects something's up with him so she tags along, despite him being cagey and weird about it.
In the car, after Sam tells Andy who is on the list, Andy suggests his addiction sponsor would be their best bet but Sam convinces her that the guy he fought would be better. Andy knows he's hiding something but doesn't say anything and Sam turns up the radio.
Back at the station, as Chloe and Dov are still struggling to find a possible explanation for Brian being targeted, Chloe vents about being frustrated with desk duty, being mad at the psycho who shot her, at Wes for leaving a blood clot in her neck, and at Dov for not fighting him on it. Dov's response only increases the tension between them.
Cut to Traci getting a call about Dex being in the hospital for a bar fight. Steve offers to pick him up since Traci is busy and though she thinks it would be really weird, she approves. That goes as could be expected, with both men taking digs at each other and Dex declining Steve's offer of a ride in favor of calling his own brother.
Shoot to Andy and Sam on their road trip. Andy tries to get Sam to tell her what's going on by suggesting silly games, but Sam rudely shuts her down. Eventually Andy gives up and angrily turns the radio back on, despite the static.
A couple of scenes later, Andy makes another attempt at getting Sam to open up with the suggestion of the game, two truths and a lie. Sam is agitated and rude, and despite Andy's persistence, he doesn't cave, to her disdain.
They arrive at the prison and a guard escorts them to the guy Larry fought. Other than describing Larry as a standup guy, he has nothing useful to say, so Andy and Sam move on. The next time the duo is seen, they are meeting with an inmate Larry worked for, who also praises him. He explains that he would be surprised if Larry reoffended because he was so excited to get out of prison and start over that he even asked for a recommendation letter, but the inmate found it odd Larry never got back to him with an address to mail it to.
Meanwhile, at the station, Dov and Chloe have found out Brian's brother Jim owned the black car before Brian and since he works in foreclosures, he would have more enemies than Brian so he could have been the intended victim. They bring Jim in and explain the new theory, to Jim's chagrin. Soon, Chris tells Traci the suspect was allegedly spotted at a bus station. Gail and Nick, who'd been on the road and realized Larry might be headed out on a bus, were already nearby so they responded and arrested him.
Shoot to Sam and Andy at the prison. When Sam steps out to take a call, Andy asks the prison guard who else is on the warden's list of people to talk to about Larry. The second name he says, the sponsor, is Jay Swarek. Just as Andy gets this information, Sam returns, elated that Larry has been arrested, which means they don't need to talk to the sponsor. When they get back in the car Andy confronts Sam about it. In the ensuing heated argument, Sam asks why they have to talk about the dark things in their past and Andy stresses that she wants to know Sam. "This happens every time, everything's going great and then life happens and you shut down," says Andy. Sam responds with a dig about her leaving to go undercover, right after Jerry died. Andy doesn't respond to this, but instead tearfully asserts that she will not go through this again with Sam. They sit in silence for a minute until Sam hands her his dad's file.
Cut back to Nick and Gail arriving at the station with Larry. As they pull in he suddenly starts banging on the window with his elbow until the glass shatters. He picks up a piece and cuts his throat in an attempt to kill himself.
Traci then informs Sean and his wife about what Larry did, and later, Sean's wife Kelly gets a shock when she passes by the board with pictures of Brian's corpse on her way back from getting coffee with Traci.
Back at the prison, Andy is going through Sam's father's rap sheet and talking to Sam about it when they hear about what happened with Larry, prompting them to go back inside and confront Sam's father Jay. Jay wastes no time trying to get under Sam's skin by being rude about Andy and Sam's mother. Eventually, Jay asks Andy if Sam ever told her how Jay ended up in prison and she says no. He takes another dig at Sam, which is the final straw that prompts Sam to give up and walk out, but before they leave, Jay says he "hopes that woman is not in any danger." So Andy asks Sam to let her talk to Jay alone and he silently agrees.
Andy gets Jay to explain that Sean pushed Larry around. When Sean started following his wife around and saw her meet another man (Brian) in a motel, he took note of the car the man got into it and asked Larry to test drive every matching car on the particular auto sale site to try to track him down. Larry brought Brian to Sean but had nothing to do with the actual attack. But Larry called Jay worried when Sean started talking about getting his wife too.
When Traci gets this new information from Sam and Andy, she, Nick and Gail race to save Kelly from Sean. But they arrive at the Harrison house to find Sean dead. Kelly is next to the body, bruised and bloody, holding a knife and trying to clean blood out of the carpet while she's in some kind of comatose state. Traci gets her to explain that he was punching her in the car on the way back from the police station and when they got inside the house and she saw a knife she felt it was like Brian left it there for her. She soon breaks down and Chris easily detains her.
Shoot to Sam back at the prison confronting his dad. He says he was scared of him, then angry, then worried he'd turn out like him but now he realizes he's free. Sam vows to never waste one more emotion on Jay and Jay only responds in a way that proves how horrible he is. When Sam leaves him, Jay is left looking like a man who's too stubborn to admit his fault and change his ways, despite the fact that he knows he should.
At the station, Traci and Dex reconcile, while Steve looks on worriedly, and then Chloe and Dov have their own makeup conversation.
Naturally, the show ends with McSwarek. Sam tells Andy the story of how his dad ended up in prison most recently: When he was wanted for a robbery and police asked Sam to corroborate Jay's alibi, Sam claimed Jay was lying even though he was telling the truth. Jay was sent to prison for six months but while there, he fought and killed another inmate and so got sentenced to 20 years. Andy isn't put off and she insists that Sam is nothing like that man and if he ever doubts it, he just has to ask her. Sam asks her if she'll always be there and Andy says, "just try and get rid of me." They exchange teary "I love yous" and are soon shown in a bedroom romantically and playfully seducing one another.
McSwarek fans rejoice, it seems they have finally overcome their biggest obstacle and might be at the beginning of a new chapter. Might they actually last as a couple this time? (If so, poor Nick).