S2, E17: With a World Trade conference in Chicago, all members of intelligence are undercover in various protest groups to determine potential threats. Ruzek (Patrick Flueger), infiltrates the Black Grid group and befriends Sierra (guest star Lucy Blehar). When she ends up dead intelligence is suspicious of an Argentine Secretary, Fabian Sosa (guest star Gary Perez). Voight (Jason Beghe) and Olinsky (Elias Koteas) question Sosa about the death and the team enlists Mouse (guest star Caleb Hunt) to help with their tech needs in tracking Sosa. Meanwhile Halstead’s (Jesse Lee Soffer) brother, Dr. Will Halstead (guest star Nick Gehlfuss) shows up in town for an undetermined amount of time and is already stirring up trouble. Also starring Jon Seda, Sophia Bush, Marina Squerciati, LaRoyce Hawkins, Amy Morton and Brian Geraghty.