S4, E15: When Lindsay (Sophia Bush) is tipped off about a grieving father looking to avenge his daughter's death, Olinsky (Elias Koteas) goes undercover as a hitman for hire. With Gang Intelligence lacking enough evidence to arrest the original gangbanger in question, the team works to reinvestigate the case. Meanwhile, Ruzek (Patrick John Flueger) comes back after finishing his undercover assignment only to have Voight (Jason Beghe) knock him down to patrol duty.