Lindsay wakes up in a strange bed, hungover and already late for work. Where is she? Seconds later (after knocking her head on the top bunk) it becomes clear - she's in her mom's house. She attempts to gulp down some coffee before running out, and Bunny insists she take some aspirin, too. Lindsay knows she's in for a long day.
Burgess feels the same way for very different reasons. It's her first day on Intelligence and it starts off with a bang - literally - as Intelligence responds to an in-progress shooting downtown. The team enters the building, guns raised, on the lookout for the shooters... but they only manage to drag one of the four into custody. Despite Voight's best efforts, the shooter refuses to identify himself or give up the whereabouts of the others. Using security footage, Halstead reenacts the shooters' precise movements through the building - these men acted with military-grade coordination. They did not murder at random, either - each victim worked on the same Lancaster Chemical case for a law firm that netted a big payday for its client. The shooters targeted them, and yet the CEO of Lancaster Chemical, Hastings, refuses to cooperate with the investigation.
On the streets, Atwater and Roman join other patrolmen on the hunt for the shooters, fanning out across the city. They locate a stray bullet casing and pull footage from local surveillance to discover that the shooters hijacked a garbage truck mere minutes ago. Atwater sprints up an on-ramp and spots the truck passing through a checkpoint. He borrows a nearby police car and, with Roman in tow, chases the truck through downtown - until the truck careens through a storefront. A standoff ensues: the police outside the building with guns loaded and the shooters inside with a dozen hostages.
Intelligence arrives to help facilitate a conversation and the lead shooter makes specific demands about cash and an escape, but when faced with a tight deadline, Voight knows the negotiation will crumble. In order to save the lives of the hostages, he decides to enter the premises on his own, unarmed. The shooters go on the defensive the second he enters the storefront and Voight struggles to keep them at bay. Meanwhile, Mouse relays a hit on the Indiana DMV database and identifies the shooter in custody as Evan Kozelek. Turns out his brother Owen, one of the other shooters, blames Lancaster Chemical for the death of his daughter. Voight uses this intel to attempt to reason with them, but Owen revolts - and gives Halstead just enough airspace for a clean snipe. After Owen drops, Voight cuffs the second shooter, Ray, and soon discovers the third shooter was never there. Quick digging into the Kozelek family points to another family member named Nick among the witnesses during the initial shooting... he fooled Burgess enough to clear him.
A clever move from Olinsky and Burgess tricks Evan into alluding to Nick's final target: Hastings, the Lancaster Chemical CEO. Intelligence rushes to Lancaster Chemical headquarters and spreads out in search of Nick. Lindsay finds him in a hallway - or rather, he finds her and places a gun to her head. In an emotional and intense moment, Lindsay dares Nick to kill her... and lucky for her, he loses his nerve. Antonio and Olinsky take him into custody and Intelligence prevents another murder, in no small part due to Lindsay's risk.
After Nick's arrest, Burgess, Roman, Atwater and Ruzek celebrate at Molly's. Burgess confesses to Ruzek that working for Intelligence is intoxicating, but he warns that Atwater will return once the board clears him. Meanwhile, after struggling with her hangover all day and listening to Platt comment about her sickly appearance (not to mention suggesting Lindsay should see a professional to talk about Nadia's death), Lindsay returns to the place where her hangover started: her mom's bar. Only instead of a quick nightcap, she finds an old friend from her past, Landon, waiting for her. They reconnect in a major way - not only do they end up on a couch tearing off each other's clothes, but on the living room table it's clear they engaged in something else first... pills and cocaine.