Chicago P.D. - Season 4 Episode 15 : Favor, Affection, Malice or Ill-Will
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 gangbanger in question, the team works to reinvestigate the case. Meanwhile, Ruzek comes back after finishing his undercover assignment only to have Voight knock him down to patrol duty.