After a luxury condo is robbed, the team is shocked to learn that not only was the family fast asleep when the crime occurred, but their teenaged daughter was violated during the invasion as well. Voight consults Jimmy from Firehouse 51 to confirm his suspicion that the family was knocked out by a powerful gas, and Lindsay, Halstead, Atwater and Ruzek go undercover to seek out the perpetrators. Meanwhile, Voight tries to help a former acquaintance get back on his feet, and Platt and Mouch receive disappointing news regarding their upcoming wedding.