The episode picks up with the quarantined patients escaping.
Intelligence rounds them up and begins their search for the man responsible for a deadly outbreak.
Dr. Marcel is forced to cut off the arm of Kwan, a resident of Ground Zero.
Intelligence begins tracking down the suspect and realizes at the same time that Will Halstead does that Dr. Seldon, the man helping them find a cure to the virus at Med, is responsible.
Jay and Voight rush over. Will walks in on Seldon as he's destroying their research samples.
A fight ensues and Seldon knocks Will upside the head.
Seldon gets away and a manhunt for him begins.
Voight enlists Trudy's help and she rounds up every police officer to be on the lookout for him.
They begin backtracking his life in an attempt to get ahead of his plan.
Everything pinpoints to June being the month that it all changed, but no one knows what happened in June.
They find the ex-wife's house and blast through a bunker in the basement to get to her.
She's visibly scared, but also slightly paranoid.
That is until they realize Seldon's federal funding for his research was cut off.
Seldon vowed to evoke an act of terrorism on the city to show them exactly why they needed to upkeep funding.
While his rented out loft reveals his next target is Oktoberfest, they realize that when he told his wife he had some unfinished business to attend too that it meant a business conference held at the building belonging to the company that pulled the money.
When they arrive on the scene, Halstead finds the suspect and attempts to neutralize him.
The doctor is spreading his disease to each person on the board.
Halstead enters the room weaponless and promises to help him prove tot these people how valuable his research is.
Seldon orders Halstead to cut himself, which he does, and then he orders him to spray something on his wound.
As he stalls, Hailey shoots a shot at the ceiling, Voight busts in and shoots the guy a few times until he's sure he's dead.
Halstead and his old schoolmate from the CDC successful create an anecdote.
Amanda, Upton's friend, makes it out of surgery and reunites with her grandmother.
The city begins living again. The Halstead brothers get a beer, Stella and Severide hook up, Voight drinks his day away, and Bowden spends time with his children.
But the most shocking moment is that Burgess and Ruzek walk off into the, er, sunset?