Roman and Burgess investigate a boy who has gone missing with a gun from his house. They find him dead behind the school. Lindsay finds a bomb in his bag. 

Platt tells Olinsky that Michelle has to be officially claimed by a parent or she'll go back into juvenile detention. Olnsky doesn't hesitate to sign the papers. 

The Intelligence team goes through the dead boy, Colin's notebook. They believe he may have a friend, Ethan, who could still be a threat. 

Roman and Burgess break up a fight at the school involving Ethan.

Lindsay and Halstead question Ethan about Colin's notebook. He denies he and Colin were planning anything.

Olinsky tells his wife that he threw away the paternity results. He tells her he wants to be a father to her. She wants him to move out. 

Mouse tracks Ethan's phone to his swim coach's house. Burgess and Roman check it out and find his car spray painted with "pedophile."

Ethan tells them that his coach abused him and Colin and that they wanted to get back at him. He says there was another boy and that the three of them were going to blow up the coach's car, but that he couldn't go through with it.

The M.E. report shows that Colin was murdered rather than suicide. 

They track Jesse to a train station but he jumps from the moving train and dies. 

They arrest the coach and he admits that Jesse wasn't supposed to kill Colin, just scare him. 

Atwater receives an invitation to Captain Whitaker's poker game. 

Olinsky and Michelle show up at Ruzek's apartment and ask to stay there.