Chicago Fire Season 5 Episode 12 picks up where Chicago Fire Season 5 Episode 11 left off. Dawson is approached by the man Casey left behind at the building fire. Later she and Brett are called to the same building where they find the landlord dead. Jay Halstead finds out that the landlord has a criminal record and identifies the mystery man as Eddie Owens.
Carly, the woman that Casey rescued instead of Eddie, shows up at the firehouse afraid that Eddie will find out she's talked. She tells Dawson that Eddie "was going to make you guys pay." Boden wants to take the firehouse out of service but Herrmann argues that would just be allowing another squad to fall into Owens' machinations.
Severide recognizes pain meds amongst the evidence recovered from the scene, and Halstead sets up a sting with the urgent care clinic that issued them. But Owens recognizes the sting and sends a decoy in his place. That enables him to set a trap for the team with bombs outside a warehouse and him lying in wait inside. Casey is able to knock Eddie unconscious and he's arrested.
Meanwhile, Brett agrees to watch Antonio's son Diego and that leads her to meet Antonio's ex-wife Laura. Laura promptly attacks Brett for trying to befriend her son, causing Brett to second guess herself. She then gets into a fight with Antonio, who tells her "you knew what you were getting into." Brett tells him he's "not ready" and breaks up with him.
Antonio later apologizes but doesn't want to reconcile. He thinks Brett deserves better, but she wants to ask him again when his life settles and he agrees.
And Dennis Mack, a friend of Benny Severide, shadows Firehouse 51 while helping Kelly throw Casey and Dawson a belated wedding reception. He offers Kelly the position of Battalion Chief in Springfield.