Picking up right where the Season 2 finale left off, Boden calls for any sign of life in the collapsed building and fears the worst when no one responds. Finally, inside, Severide answers and stirs the squad back into action. They soon find Mills buried under rubble, his leg broken. Otherwise, each firefighter is accounted for - except Dawson and Shay. Severide frantically searches the building and discovers Dawson administering CPR to a bloody and pale Shay. He joins Dawson in the attempt to revive their friend and teammate, but it's too late. She's not breathing.
Six weeks later, Firehouse 51 remains somber as the squad continues to mourn the loss of Shay. Mills, his leg healed, returns to duty and learns that Severide has yet to report back, the lieutenant's whereabouts unknown. But other news piques Mills' interest - Newhouse informs him that he landed a lead on his paternal grandfather and provides Mills with a family tree. His relatives live in Joliet, a Chicago suburb not too far away.
Dawson, too, keeps her emotional distance at the firehouse, almost offended when the new paramedic, Sylvie Brett (aka Shay's replacement), arrives at the station. After a rocky start, Brett proves her worth when it counts, improvising to save a woman in the heat of an intense rescue. This impresses Dawson and suggests the paramedic team will be in good hands when she leaves 51 for Firehouse 105.
Speaking of, Welch and the 105 squad join Casey and Truck 81 at a fire truck fill station. A call comes through before the rival houses get too testy and they race each other to the fire. Truck 81 arrives first and Casey heads inside for a daring rescue. Afterwards, Welch taunts Casey and the two nearly come to blows before Boden breaks up the ruckus. The chief challenges Casey to be a better leader.
This spurs Casey to seek out Severide and he finds him chopping wood at a remote cabin, with no interest in returning to the "cursed" firehouse. Casey allows his friend to say his piece, then counters with a list of lives they saved over the years and reminds Severide to focus on who they rescue and not who dies.
After six weeks of bottling her emotions, Dawson finally cracks during a counseling session and admits that she and Shay traded places at the last second - that beam should've killed her, not Shay. The guilt of surviving kept her silent, but now she cries on the counselor's shoulder, the sadness overwhelming.
Annoyance overwhelms Boden in the meantime as he and Donna accompany Mouch and his new lady, Desk Sergeant Trudy Platt from Chicago P.D., for a double date. Donna understands her husband's frustration with the often awkward situation, but insists that the outings are good for his work/life balance. He takes her advice and agrees to join Mouch and Platt on another double date at a Led Zeppelin cover band concert.
Later, a clean-shaven Severide lingers outside his apartment until Casey encourages him to head inside. They go through some of Shay's belongings and Severide announces he can't live in the house without her. Casey offers up his and Dawson's apartment; Severide agrees. Before they leave, he rediscovers a DVD he and Shay made when they first became roommates. He watches the short video alone, tears in his eyes, as he sees Shay speak the mantra of their friendship, their most important rule - to be there for each other. Always.