The team are called out to a brutal five person murder at a Chicago fitness suite. The team questions a list of suspects and manage to find one of the gun men at the gym, but he won't let them know anymore. Voight isn't best pleased and tries to get the answers he requires in order to find the other people involved.
When the other two men are found, Voight must go into the building and surrender to them. He finds out that the reason behind the attacks is that Owen's daughter was killed by chemicals found in water from the company, so they are avenging her death in any way they can.
Voight tries to reason with them, but Owen is killed and the other guy taken to custody.
The team realizes that Nick was also involved and have to go find him. Lindsay finds him and tells him to shoot her, but he doesn't and he is tamed.
Lindsay goes back to her mom and she tells her she met a friend from her past. He shows up and the two of them take drugs and sleep together, how will Halstead react to this shocking news?