It's a rare night at Firehouse 51 - slow and quiet. Herrmann, Otis and Dawson discuss modifications to Molly's with their newest partner, Shay, while Cruz cooks up some less-than-mediocre chow and Katie relays some big news to her brother: she's moving to New York to open a restaurant with her boss. The decision fosters some guilt, considering Katie's new romance with Otis and her relationship with Severide. Rather than give Otis the news, she peps him up by whispering, "tonight's the night," and the smile on his face says everything.
The first major call sends 51 to a car wreck and a teetering power line. Just as they discover an injured body and a passed-out drunken driver, the power line crashes to the ground - sending the entire grid into total darkness. Once the scene's secure, Boden offers people without power safe haven at Firehouse 51, where a generator keeps the station warm and bright... until the furnace busts. As Casey and Severide work on a fix, Severide quickly realizes his friend continues to suffer from memory loss, even forgetting names of common tools. Their brief spat pales in comparison to what happens next - a fight breaks out between two civilians in the kitchen. The instigator, a punk named Vince Keeler, snags a knife and points it at Severide, a move he regrets after Severide easily disarms him and tosses him into the cold.
After the dust settles in the firehouse, Detective Lindsay from CPD makes a surprise visit to clue in Boden, Casey and Severide on a pressing update: A trusted informant slipped her some intel on that Keeler guy - turns out he's got a serious reputation and is making noise about killing a fireman tonight, pissed at how Severide humiliated him. She reiterates the warning: stay away from Keeler.
With dozens of additional mouths to feed, a panicked Cruz sends Clarke on an errand for more food at a corner store, but while he's gathering supplies in the stockroom, would-be looters enter and stab the owner. Clarke scares them off with a bat and improvises a way to stop the bleeding. Just as the owner appears stable, the thieves return with friends - all armed and ready to fight off Clarke. Luckily for everyone, CPD officers Atwater and Burgess pass by at the right moment and defuse the conflict.
Back at the station, Boden's eyes perk up when Donna arrives. The power's out in her building, too - not that Boden would mind either way. While they try to steal a moment together, Otis and Katie sneak off into the showers to make out - a choice that backfires when both Boden and Shay stumble upon them. Otis seeks out Boden for an apology, except Otis finds the chief preoccupied in his office - with Donna. At least Otis knows Boden won't pursue reprimand...
Meanwhile, Dawson decides to help out Shay and Rafferty due to the blackout, but bristles with her replacement during a brutal carbon monoxide poisoning call. Later, an older woman's heart attack at the station leads to another collision between the ladies, only this time Dawson lets Rafferty run point. Once they revive the woman, Dawson gives Rafferty her due and passes her the EMT torch - it's her purview now.With the chaos somewhat contained, Detective Lindsay returns with another update - word is, Keeler's got an inside man with sights on murdering Severide inside the station. She circulates a photo of the suspect and a thorough search turns up nothing - until Casey notices the man slinking away in the shadows. Casey stops him short, but not before he takes a hard punch. Only later does the incident make him think twice - when he spots blood dripping out of his ear.
As the morning sun peeks over the horizon, the power grid flickers back to life, sending all the civilians back to their homes and relieving the squad from their shift. Walking back to her apartment by herself, Katie chats with Otis on the phone, excited to meet up, when she's interrupted - by two thugs! A sack over her head muffles her scream and before anyone can notice, she's yanked off the sidewalk and tossed into a van.