After a night of lovemaking, Dawson and Casey chat about their bold decision. Casey admits he regretted not choosing Dawson last year when he had the chance. At the station, Dawson arrives to meet another temporary replacement for Shay - the neurotic, pessimistic Erik McAuley. She dreads working with him, but things look up when Casey surprises her in the back of the ambulance, ready for a little on-the-job nookie.
The timing's bad, however, as the alarm sounds and sends everyone out to rescue a cop clinging to the edge of a rooftop. Casey's quick thinking - and a cool cuffing device manned by Mills - saves the man's life.
Still stinging from the robbery, Otis and Severide complain about the lack of news. Shay contends she's done everything she can, but now she's late for her shift. Severide pleads with her to rejoin 51 - Shay stays firm. Later, Shay leaves an emotional voicemail on Devon's phone, pleading with her to return everything she stole. Alone and depressed, Shay skips out on her shift and instead makes her way to a downtown bridge over the Chicago River. Is she suicidal?
Across town, Leon works the streets undercover, back with the Insane Kings. Cruz drops by to inform him about his wedding to Zoya, but Detective Olinsky from Chicago PD interrupts. This is a sensitive operation, no contact allowed. Antonio echoes the statement later when he meets with Cruz and Casey at the station. Turns out Insane Kings' leader, Doc, scheduled a sit-down with Leon, and it's the perfect opportunity to get a confession. Cruz flips - Doc is dangerous! - but Antonio assures him that Leon's in good hands.
Frustrated with his living situation, Otis fantasizes about leaving the apartment and Mouch pipes in with an interesting suggestion - why not force them to throw you out? While he considers this, Katie arrives at the station with lunch for the squad and announces herself as Severide's sister. Her food's a big hit and when Severide returns her supplies to Katie's house, he learns she's graduating from cooking school and her mom can't attend. Could this be a brotherly opportunity?
But Severide has more on his plate after Boden lets him know Shay never showed at Station 25. Where the hell is she? He checks her usual haunts to no avail and settles on a last resort: Dawson. He demands she contact her old friend. It could be a tall order, though, since Dawson and Casey can't keep their hands off of each other. The two lovebirds meet in the shower for some privacy and things get hot fast. But their rendezvous is noticed by McAuley - and Mills.
Luckily, Mills must contend with his own distraction: Isabella. He attends another swanky mixer with her, only to be cornered in the coatroom by Senator Wheeler - who reveals he shares the same suspicions about Isabella's intentions. This could spell trouble for their rekindled romance - an issue fellow fireman Clarke knows all too well.
Desperate for some guidance, Clarke confides to Hermann about his wife's ongoing affair with another man. How can he make this work? Hermann warns him that could be the problem. Maybe it's time to stop trying to make it work...
Leon's undercover assignment finds him in a similar situation as he drives down a highway with gang leader Doc while Cruz, Antonio and Voight follow them and listen in on their conversation. Right when Leon manages to get Doc to admit to first-degree murder, a rival gang rolls up - and riddles their car with bullets! CPD pulls the bloody victims out and finds Leon somehow unscathed.
Cruz assumes the worst is over until Antonio gives him the real lowdown - they need to move Leon out of the city to keep him safe, and it could be permanent. Sad to see his brother leave, Cruz's fury quickly turns to tears and the two hug goodbye.
To the shock and joy of his newfound sister, Severide shows at Katie's graduation and then throws her a surprise party at Molly's. While Katie enjoys a shower of congratulations, Isabella finds Mills with some big news - Senator Wheeler offered her a job. She suspects Mills helped her cause and would like to return the favor. He smiles and suggests they leave.
Nearby, Otis mans up to Severide and tells him he can't stay in the apartment anymore. Severide's bummed, but understands - a sentiment Otis hopes to duplicate after Zoya instructs him to deliver bad news to Cruz: she wants out of the marriage.
Meanwhile, Dawson locates Shay on the bridge. Sad to see her friend on the verge of suicide and feeling guilty over their petty fight, Dawson finally says she's sorry and Shay agrees to return to 51. The whole gang meets up, excited to see Shay again, when they're greeted by an all-business McLeod. She's drummed up a liability charge that solidifies 51's imminent closure. Everyone has one final shift together, and then that's it. No more Firehouse 51.
Dawson and Casey's romance escalates while Shay hits rock bottom and goes MIA.