Burgess wakes up to find that her sister never came home the night before. Nicole calls and tells her she's in trouble.
Burgess and Ruzek track her phone and go to look for her. They find her in the train station. She tells them she's been raped. She doesn't remember what happened.
Intelligence investigates. They question her friends and get security footage from the bar. They get a warrant for a suspect and find pictures of Nicole in his house. He says he was hired by her ex husband.
One of Nicole's friends, Denise, never came home either, and they haven't been able to track her down.
Nicole tells Burgess she remembers balloons and sparks from the train.
Voight goes to a seminar of a suspect named Goldwin. He teaches men to "push through" a woman's resistance.
Halstead and Lindsay find Denise dead. She died of hypothermia, but has no signs of sexual assault.
They suspect a couple of Goldwin's "students" and Voight puts Burgess and Lindsay undercover. The guys pick them up and they go home with them.
Burgess ignores Voight's rule not to drink anything and takes the shot she's offered. She lures one guy to the bedroom and then beats him to a pulp.
The guys are arrested and Burgess gives closure to her sister.
Voight makes sure Goldwin will not be teaching any more seminars in Chicago.