Gathering everyone, the Chief introduces Gavin and Presley, two visiting firefighters from Toronto who will be observing the firehouse over the next couple of shifts. Before formal introductions can be made, the sirens echo across the kitchen. The crew arrives at Logan Square Station where an energized third rail incident has stopped a train. As the Chief works to shut off the power, Severide, Shay and Dawson free a young boy named Jacob, who's trapped in the tracks, while Mills goes off in search of Jacob's sister Kayla, who was also hit. Minutes later, Casey comes upon Mills in the darkened train tunnel, silently kneeling. He looks ahead to see the body of a girl face down in the tracks, body parts scattered all around the tunnel. Casey solemnly calls for two body bags and puts his hand on Mills, who's speechless.
Later that day, the guys are recuperating in the kitchen. The Chief reminds them that it was a tough scene today, and there's no shame in reaching out to speak to someone. Sitting across the table from Gavin and Presley, Moutch interrogates them about their home country. Clearly, he's got some beef with Canada. From across the room, Mills watches the group. How can they act so normal after the horrifying scene they've just witnessed? Outside, Shay is sitting with Clarice. It's an awkward situation; Clarice is overjoyed to be spending time with Shay, while Shay reminds her that she's got a husband to go home to. Thankfully, they're interrupted by the siren, and Shay and Dawson race to a house where a young girl, Grace, has just suffered a seizure. They rush her and her mother Nina off to Lakeshore Hospital, where Nina informs Dawson that Grace has celiac disease and a history of unexplained seizures. Dawson senses there may be something else going on.
At the firehouse, Severide watches a zombie-like Mills stand over a pot of soup. When Mills steps outside to throw out the trash, he breaks down, bracing himself against the gate as he's tries to stifle his sobs. Casey asks the Chief for permission to leave for an hour and asks him to keep an eye on Mills. Dawson and Shay, back from the hospital, ask Casey if Hallie can possibly look into Grace's medical file for them, but Casey informs them they're currently on a break. Shay calls an old friend at the hospital, and they learn that not only did Nina lie about the celiac disease, but also Grace has been in the emergency room twice in the last six weeks for seizures. Sirens blare overhead, and the crew is called to a scene where a man had a heart attack while collecting branches up in a tree.
Running up the ladder, Severide hooks the pulley onto the man's vest and tries to lower him down, but his shoulder seizes up. Mouch and Otis run down to grab hold of the pulley and slowly lower the man to safety. Upon inspection, Dawson realizes that the victim is diabetic, and they work quickly on his blood sugar. The man's eyes spring open; thankfully, he'll be just fine. Away from all the action, Casey arrives at a quiet cemetery and makes his way towards a gravestone in front of which three people stand. They're introduced as Casey's sister, Christie, her husband Jim and her daughter Violet. It's clear that they haven't seen each other in quite some time, years even. Casey steps up to the gravestone: it belongs to Gregory Casey. Today is the 15th anniversary of his death.
In his office, the Chief tells a stricken-looking Mills that they've all been there. Right now, Mills doesn't have anything to compare the experience to, but he will. It's what the job is. The Chief advises him to take the shift off and talk to someone. As Mouch folds his laundry, a gleeful Otis comes up to him and informs him that the Canadian firefighters are actually from Cleveland, and Herrmann made them play a joke on Mouch. Mouch and Otis plan to get Herrmann back and beat him to the punch. Outside, Shay's got an unwanted visitor, Clarice's husband Daniel. He's livid that Clarice has been staying with Shay a couple of nights a week, but Shay coolly informs him that it's not her problem. Sighing, she assures David that she's fairly certain she can get Clarice back home today.
Mills seeks comfort from his mother, but it doesn't quite go the way he planned. Instead of offering advice, his mother begs him to quit and leave all this behind. She saw what it did to his father: he became a block of ice, a shallow outline of a person because of all the terrible things he'd seen. At the firehouse, Severide's on the phone, begging for a refill of his medicine. Shay and Dawson return to Lakeshore where they learn that Grace seized because of swallowing turpentine, which her mother claims to have left out accidentally. Understanding that Grace's mother has been going from hospital to hospital to hide the abuse, Dawson reaches out to DCFS.
That night, the Chief has an unexpected visitor: Ingrid, Mills' mother. She informs him that Mills is not like his father; and he so badly wants to live up to his memory, but that's not the life she wants for him. The Chief believes the choice to stay with or leave the profession is solely Mills' decision, but Ingrid icily tells him that he will not be returning. When the Chief runs into Severide in the locker room, he asks him what's the matter with his arm. He's suspicious, but Severide assures him he just tweaked it a bit lifting weights.
Horrified that the DCFS didn't think there was a case with Grace, Dawson drags Shay to the Wilkinsons' doorstep so that she could have a word with Nina. Shay tries to stop her, but Dawson is relentless. When Dawson desperately tells Nina that she's poisoning her child and needs to see a doctor, Nina accuses Dawson of fabricating lies and tells her she'll be hearing from her lawyer. Back at the firehouse, the Chief really hands it to the partners. Shay needs to keep her hotheaded partner under control, and if Dawson doesn't cool this crusade she's on, it's her job that's on the line.
The following morning, Severide stops by the restaurant looking for Mills. Mills' sister informs him that Mills went on a drive a few hours ago, but no one's heard from him. At her apartment, Shay firmly tells Clarice to go home to her husband. Clarice is the one who left her. Now that she's pregnant and married, she can't just waltz back into Shay's life. Clarice understands how badly she's hurting Shay and makes her exit. Later at the firehouse, Mouch pulls a crazy prank in which buckets of water rain down on Presley and Gavin in front of the entire firehouse. That's what they get for lying about being Canadians! Except that they're not lying, and Otis is the one who fooled Mouch. The Chief isn't happy with Mouch and Otis' ridiculous antics, but before he could say a word, the sirens ring for the ambulance. The destination? Grace's apartment.
Shay and Dawson rush into the apartment expecting to see Grace, but instead they find her mother on the couch, barely breathing after taking an entire bottle of Doxepin. At the firehouse, the Chief reprimands Otis for the stunt he pulled. Gabriel and Presley didn't drive 10 hours to be the butt of a joke, and the Chief orders Otis to apologize on behalf of the entire department. As Otis heads over to the two, he overhears Mouch finally telling the story of why he has it in for Canada: he once gave $5,000 to a Russian woman who he had fallen in love with in Toronto so that she could facilitate her American citizenship, but he never saw her again. Turns out the government agents at the Canadian Anti-Fraud center didn't help Mouch, but merely chuckled at his expense. Gabriel and Presley apologize on Canada's behalf, much to Mouch's appreciation.
At Lakeshore, Shay, Dawson and the Chief are interrupted by Mr. Wilkinson, who accuses Dawson of being the one to blame for his wife's suicide attempt. Later, a weary looking Grace steps up to Dawson and, with permission from her father, admits that her mother forced her to drink turpentine. Dawson was right all along about Nina's manic tendencies. That night at his apartment, Severide receives a package with some more of his mysterious medicine and a note that says, "You owe me." He immediately pops a few tablets in his mouth, shoulders sagging with relief. When Shay's southern fling shows up looking for her earrings, a shared look between her and Severide sends them straight to the bedroom. After all, they're both just looking to keep things simple and, most importantly, keep things fun.
Picking up Mills after his shift at the restaurant, Casey brings him to an unfamiliar house. They're greeted warmly by a man named Gary and, soon after, his bubbly daughter Sophie, a charming young woman with terrible burns all down the back of her arms and neck. As she's showing Mills and Casey her most recent trophy, Mills can't help but stare at her badly burned skin. He apologizes, but Sophie doesn't mind. What's some bad burns when, thanks to Casey who pulled her out of the fire, she's alive and well? It dawns on Mills that in their line of work, for every life lost, there's a life saved. And that, perhaps, is the most important thing of all.