When both Truck and Squad respond to a house fire that leaves a woman dead Casey and Severide are approached by the victim's sister who believes that the victim's husband set the fire to kill his wife. Casey believes she could be right based on evidence at the scene but Severide, who had to console the grieving husband, disagrees.
The husband calls Severide for a meeting and Severide uses the opportunity to press him on his alibi, finding a city camera that proves the husband wasn't home when the fire started. OFI thus doesn't press charges but Casey insists that he'll prove the man's guilt regardless.
Elsewhere Hermann goes on the offensive when he finds out that someone has been tagging the firehouse, going so far as to stake out the front of the building. When he spots the tagger on the street Hermann gets out of the truck to tackle him and then sets him straight.
Dawson and Brett help a teenager who fell out of a tree during a homecoming proposal. Brett finds the girl he was trying to impress at Chicago Med but it turns out she already had a date. To add insult to injury cell phone video of the accident has been posted online. So Brett saves the day by being the backup date.
And Platt's friend makes an offer to buy Mouch and Brett's manuscript but Brett mistakenly leaves it in the copier. That brings their book to the attention of the CFD's Publicity department, which is not amused and threatens them with termination. Mouch drops the publishing idea but comes home to find Trudy has started a book club with the novel.
The episode ends as Donna tells Boden that someone's been watching their house, and Boden discovers that it's his son James.