Serial killer (and ardent YouTube poster) App the Ripper slices up another victim outside a busy nightclub and skirts away before the team - already on the case - can catch him. Ironside spots a couple fleeing the scene as the vic dies in his hands. The death troubles him but so does that mysterious couple - why would two eyewitnesses bolt if they could ID the murderer?
With no leads save a few security camera images, Ironside takes a night off to hook up with a recent admirer. A casual conversation triggers a return to an old case of a missing girl he and Gary never tracked down. Aged about eight years, she looks just like one of the MIA eyewitnesses.
Sure enough, the parents give Nicole a positive ID of their daughter - not to mention the man with her, her uncle Chris. Holly and Ironside visit the couple's vacated apartment and discover one room in squalor and another - clearly Nicole's - the epitome of spoiled indulgence. Strange relationship for a kidnapping.
They check up on Chris' workplace - where he hasn't shown in two days - and interview a colleague who gives glowing reviews of Chris as Nicole's "brother," but Holly thinks she's holding back. Ironside sees it a different way - what if she's protecting Chris? Sometimes people break the law for the right reasons. He leaves a secret note for Chris, confident he'll turn up.
Hours later, Chris appears inside the precinct and it's just as Ironside figured - Chris kidnapped Nicole to get her away from a sexual predator, her stepdad, Carter. Nicole confirms the story in an emotional interview but reveals she didn't see App the Ripper's face during the murder. The only thing she noticed was a missing finger on his hand.
It just so happens that Virgil and Teddy visited an accommodating nine-fingered knife expert while pursuing a string of dead-end leads on App the Ripper's weapon of choice. They pay him another visit, and this time he's not so friendly. He pulls a knife and Teddy has to talk him down just long enough for Virgil to clock him unconscious with his gun.
Ironside sees to Carter's arrest and helps connect Nicole with Chris' co-worker for a temporary home. The same help can't be given Chris because he remains guilty of kidnapping. Since the details make it a federal case, Ironside helps the only way he can - by lying to the feds about his own police misconduct, in turn sabotaging their ability to prosecute. They let Chris go, and Ironside rolls out of the station knowing he made the right choice.