In the middle of the day, a fire that started in a trash can near the building is raging through a machine shop. The scene is more dangerous than most - it's full of cylinders and oxygen tanks. Otis spots Ernie, the young boy from the firehouse, watching the men a few feet away. The team works quickly to keep the fire under control, but while Vargas and Severide attempt to break down the roof, a fresh batch of smoke sends Vargas to the ground in a terrible coughing fit. He's immediately taken to the hospital. When Severide spots Ernie, he turns on him, asking him if he torched the place. Chief Boden intercepts and tells Severide he's handling this, but Severide won't have it. "That's two fires, two dumpsters, same kid!" he yells furiously at the Chief.
The next morning, the Chief explains that Vargas inhaled magnesium shavings, which resulted in an upper respiratory tract infection and severely burned lungs. He's out of the hospital, but not out of the water. Privately, the Chief warns Severide that if he ever speaks to him that way again, there'll be a price to pay. Severide self-medicates with painkillers. Meanwhile, Shay asks Casey to help fix Dawson's kitchen window, since her landlord isn't the most reliable guy. Taking matters into his own hands, Boden stops by Ernie's house, asking him if he knows anything about the machine shop fire. The sooner Ernie speaks up, the sooner the Chief will be able to help him out. Ernie's shaken and silent, but he agrees to stop by the firehouse by 2pm with his grandmother.
Called to a bank building, the team deals with an elevator accident. As Casey lowers himself into the shaft, he discovers that the elevator is empty except for a huge safe box. The thief, Mark, stands hovering on a ledge, but Casey carefully gets him to safety. Outside, Cruz receives a call from his brother Leon, asking him to bail him out of jail, while Dawson makes plans to cook Casey dinner to show her gratitude for his handy skills. At the firehouse, everyone happily welcomes back Vargas, who has to take an inhaler four times a day, but is otherwise feeling good. In his office, the Chief informs Casey and Severide that Vargas is being put on long-term disability. The CFD doctor explains that the test results show chronic obstructive pulmonary disease, a slow deterioration of the lungs and skeletal muscle dysfunction. There's no way he'll be able to handle the demands of the job.
In the locker room, Casey and Severide break the news to a heartbroken Vargas. They try to convince him it's not all bad - he'll still be getting paid of his salary. Casey asks Severide if he's gotten his shoulder checked out yet. Severide firmly tells Casey that it's his neck, and that he's fine. The squad is called to a small car accident involving a power line, where Severide saves an attractive young woman. Meanwhile, Cruz bails his troubled brother out of jail, telling him that if he keeps on going this way, he's going to die in a gang. Cruz wants Leon to move in with him. At the firehouse, Mills flirts with Dawson, and it doesn't go unnoticed by Shay. As the clock nears 2pm, Herrmann and the Chief await Ernie's arrival. Herrmann advises the Chief that he's not doing anyone any favors by waiting for Ernie to do the right thing - these kinds of issues don't just fix themselves.
Renee Royce, the woman from the car accident, arrives at the firehouse with a box of chocolates for Severide and asks him out for coffee. Severide gently turns her down, telling her it's a well-known phenomenon that people who are rescued become attracted to the men who help them. In the locker room, as Mills watches Vargas pack up, Vargas offers some essential advice: when an opportunity comes, grab it. As Vargas makes his exit, the entire firehouse hands him his firefighter hat, signed and inscribed, and hugs him goodbye. Later, the Chief heads to the fire investigation office and hands Ernie's files over to the head of the department.
At a sketchy convenience store, Cruz sits down with Rocco, a gang leader, and tells him he wants Leon out. Rocco wants $10,000 for the trade, but Cruz stands his ground and refuses. Since their relationship goes back a while, Rocco agrees to let Leon go. At the firehouse, Casey and Dawson flirt while picking out a window. As they pass one another in the hallway, Shay accuses Severide of still taking painkillers, but he advises her to stay out of his business. Outside, Renee is waiting for him. She's not taking no for an answer and has made dinner reservations for tomorrow night. Severide is both flattered and intrigued.
Vargas returns to the firehouse, wanting to be in the company of his friends, but it's clear that there's been a shift. That night, the Chief arrives at yet another fire started by a dumpster flame. He spots Ernie, and after receiving a new address from the investigator, the Chief goes to investigate. He uncovers a terrified-looking Ernie under the care of someone he calls his Uncle Ray. The two flee before Boden can ask more questions. Later, the Chief learns that "Uncle Ray" is Raymond Martin, an arsonist who served eight years in prison. Walking out of the firehouse, Severide notices that Vargas stayed the entire shift. Old habits die hard. He invites him on his date, and though he apologizes to Renee afterwards, she kisses him and tells Severide he's a good man.
As Cruz is smoking a cigar outside his home, an Escalade speeds down the street and throws out a badly beaten Leon, his face covered in blood. Apparently, the gang doesn't like family members shoving their noses in the gang's business. At Dawson's, Casey has successfully installed the window, and the two are sharing a few beers. As they get close to take a picture for Dawson's wall of fame, sparks fly and the two almost share a kiss but are interrupted by Casey's phone. It's an emergency, and Casey runs out of the apartment. He arrives at a rooftop, where a bewildered Severide is trying to talk Vargas down from a ledge. Casey assures him that Firehouse 51 will always be made up of the men who walked through its doors, regardless of where they are now. The firehouse will always be his home. Listening to his friends, Vargas makes his way off the ledge and collapses into their arms. Casey and Severide stare at one another, stunned and disturbed by what they've just witnessed.