A familiar face is back as Kylie joins the squad for a shift.

Violet is unimpressed with a story Rosie tells about having an affair with a married man.

The crew responds to a massive fire at a music studio. There's one victim and he tells Kelly that he doesn't know what caused the fire.

The studio owner shows up to the scene and only shows concern for his employee, thanking the firefighters for their work.

James asks Kylie about when paychecks will go out, and she tells him it will be a few weeks since it's his first check. He's concerned about that wait and she offers to talk to the payroll desk to ask about an advance for him.

Severide and Van Mater are trying to determine the cause of the studio fire and smell something strange. They discover a charred human body in the rubble of the studio.

The studio owner tries to fill in blanks for Severide and Van Meter. He tells them about a client he suspects may be responsible for the fire.

Mouch's patient, Kathy, shows up at the firehouse for a visit but disappears into the station when Mouch steps away.

Severide recruits Stella to help him research the fire suspect. Severide decides to go looking for him on his own.

James admits to Kylie that he's worried about losing his mom's home because the friends he rented it to haven't paid rent, and he's behind on the mortgage. Kylie tells him he should tell Boden and ask for help.

Severide locates Antoine, the fire suspect, but an alarm is pulled in the bar, and Antoine runs. PD shows up and takes Antoine into custody.

Antoine is questioned by PD and Severide. Antoine insists that he had nothing to do with the fire.

Severide tells Van Meter that he doesn't think Antoine is the arsonist. They agree to keep searching for answers.

While making food with other firefighters, Mouch finds out that Kathy is sleeping in the bunks. She tells him that she's having a hard time being away from him and Mouch is concerned about the boundaries that have been crossed.

Mouch makes Kathy leave the station but promises he'll see her at her home visit.

Rosie and Violet try to get to know each other while driving the aid car. They get a call about an injured child and respond.

They reach the home and go into the basement to find an child on the floor with a gunshot wound. They look up to find another child pointing a gun at them.

Rosie tries to talk down Theo (the child with the gun). Theo is scared that he'll be in trouble again if the other kid is dead. Rosie manages to keep Theo calm by telling him a story of a time she was scared while Violet works on the injured child.

Theo finally hands Rosie the gun. Violet is able to save the other child who is now stable.

Severide finds that the expensive equipment has been cleared out of the studio before the fire and now he and Stella are concerned that the fire was arson for profit.

Boden confides in Kylie that he's worried about James and she breaks her promise to not say anything to him because she wants to help.

Severide gets to the bottom of the arson. It turns out that Wyatt, the person who died, set the fire to cover up stealing the equipment. He planned to sell it to get out of debt.

Mouch arrives home and finds Kathy sitting at the dining room table with his wife. He is visibly panicked.

Mouch asks Trudy if he can help her cut the cake but Trudy tells him not to be ridiculous. Trudy tells him that's ridiculous and leaves him alone with Kathy.

While Trudy is gone, Kathy confronts Mouch about being rude to her. He tells her he is worried she's gotten the wrong impression from him.

Trudy comes back and offers to share the cake with Kathy. Kathy declines, and Trudy slams her down and detains her. She found a razor blade in the cake and is placing Kathy under arrest for assault with a deadly weapon.

Trudy explains to Mouch that she had an officer run a background on Kathy. She learned that she had previously been in a facility after attempting to commit murder.

Mouch is stunned and stands with his mouth agape as Trudy leaves with a handcuffed Kathy.

Violet asks Rosie if she'd like to stay on for a few more shifts.

Severide takes the recovered recordings to Antoine, who is very thankful. Antoine promises to send him a copy of his album.

Boden shows up at James's mom's trailer and starts helping him clean up the mess left by the tenants.