On November 15, 1995, Barbara Taggert burned to death and her daughter Lisa disappeared. Years later, when Spencer Taggert's old partner Bill Harris rescinds his alibi, the squad takes another look at Taggert, wondering if he may have been the one to kill his wife and kidnap his own daughter. When they approach Barbara's father, Gordon Shaw, he defends Taggert all the way, insisting there is no way that he would ever hurt his wife or child. His reaction isn't the only problem; another little girl, the same age as Lisa, had disappeared from the area shortly earlier.
Jill and Kai watch from behind the glass, and Jill swears that Taggert looks like an innocent man. Taggert is reduced to sobs when they question him about his alibi the night of the fire, until Shaw and a lawyer show up. They take Taggert away, and just as Ali is getting advice from Nick, Taggert shows up to admit that he asked Bill to lie about where he was the night of the fire -- he was actually with a married woman instead of being at the office.
That night at a bar, Nick runs into Taggert's vehicle, breaking his tail light. He goes into the bar to 'confess', and offers to pay Taggert for the damages. When Wendy Musgrave comes in the next day to support Taggert's alibi, the detectives are disappointed, but Logozzo is thrilled when Harris agrees to a polygraph. Ali informs her partners that Nick is currently on the case. Lloyd and Tony watch Harris perform on the lie detector while Nick asks Ali what Taggert is guilty of. She declines to tell him, but when he realises there is something wrong, she admits to him that her mother has tested positive for breast cancer.
Nick and Spencer head out ot the bar that night, and Nick tells him that he 'provides a specific service for his clients'. Spencer reveals that he was gypped by a former business associate, and wonders if Nick can help him out. To gain Taggert's trust, Nick starts a fight with the boyfriend of a woman Taggert was flirting with, and when the cops come in, Nick is pepper-sprayed and arrested. Luckily this cements his friendship with Taggert, but Nick is frustrated at Taggert's insistence that he was not connected to Barbara's death or Lisa's disappearance. Back at the squad house, Nick muses that whenever Taggert drinks, he is turned on by violence.
The squad sets up a situation where Nick needs to shake down a man and get money from him. Nick goes overboard, pressing the barrel into the neck of the man he's shaking down. Taggert is impressed, and wants to hire Nick to kill his father-in-law, Gordon Shaw. The lead about the other missing girl also fizzles, because it's believed she was abducted by a non-custodial parent. At the station, the other man involved in the shakedown confronts Nick, and the other cop (from vice) reveals that he has never been treated the way Nick treated him, and he and Nick fight. Later, Nick returns to Ali's office to tell them Taggert says it's now or never.
Ali goes to Shaw to tell him about his son-in-law's plans. At first he doesn't believe her, until she plays the tape of Spencer saying it in his own voice. Nick is with Spencer, who produces a gun that he wants him to use, and he's not willing to take no for an answer. The two return to a hotel room, where Nick waits for Ali to call and tell him that it's all set. Nick pretends he got a call from a friend who says that Spencer is the main suspect in Barbara's murder, and confronts him, demanding to know whether Spencer is a liar who murdered his wife, or if he's innocent after all. Nick continues to taunt Spencer, telling him he thought they were friends and he trusted him. Then Nick turns the tables on Spencer, accusing him of being a cop.
By this time, Spencer is not only angry, but terrified. Nick has him strip down to prove he's not wearing a wire, then again wants to know if he killed his wife, because Spencer could be caught. He says no way, and Nick has him drop his drawers too. With Spencer standing there naked, Nick again asks Spencer if he killed his wife, to which he responds, 'Yeah, I killed her. I hated the bitch.' And he even admits to enjoying it. Spencer brags that they will never figure it out. He staged a break-in, strangled her, then set a fire in her potter's kiln. By the time the fire actually started, he was already with his girlfriend.
Nick asks about the kid, and Spencer laughs insanely as he admits to having killed little Lisa as well, burying her where the cops would never find her. He shows absolutely no remorse. After deadening the wire, Nick starts to tell Spencer about ways to destroy bodies, and says they'll make sure that no one can ever identify his daughter's body. He makes Spencer a deal -- show him where the kid is in exchange for Nick killing Gordon Shaw. He claims it's the only way to make sure they will trust each other forever. In the other room, Ali finally decides it's time to arrest Spencer, but he and Nick are gone.
Spencer and Nick drive to a spot in the woods where Spencer claims that his daughter Lisa is buried. Shovel in hand, Spencer begins to dig up the body of his murdered daughter. Nick says he has to go back to the car for his chemicals, and takes advantage of that time to call in the rest of the cops. Ali is furious that Nick went out of protocol, until he tells them where he is. The squad shows up, and Tony reads Spencer his rights as Kai cuffs him. Spencer tries to lunge at Nick, but he's restrained. Ali wonders aloud how far Nick would go to make his case, and he tells her as far as he has to -- just like her.