Chicago P.D. - Season 4 Episode 15

When Lindsay is tipped off about a grieving father looking to avenge his daughter's death, Olinsky goes undercover as a hitman-for-hire. With Gang Intelligence lacking enough evidence to arrest the original gang-banger in question, the team works to reinvestigate the case. Ruzek comes back after finishing his undercover assignment only to have Voight knock him down to patrol duty.