Linda Russo is found dead in an alley two blocks from her home. Castle believes this could be the work of the notorious Triple Killer, who would strangle three women a week and arrange each body as if they were in a coffin. Paul McCardle, the former horticultural technician at the victim’s workplace, had a little obsession with Linda. A horticultural technician, by the way, is a guy who waters the plants. The nervous, sweaty suspect alibis out and Captain Montgomery informs the team that the Triple Killer has struck again.
Kim Foster has the same distinguishing strangulation wounds as the first victim. She was found in her apartment, which matches the previous M.O. of the Triple Killer. Beckett realizes that Linda Russo identified the suspect four years ago during a call to a tip line. That's why he dropped out of sight. That's also why Linda was blindsided in an alley. The killer was on Linda's radar and could never gain access to her apartment.
Fingerprints on a nametag found at the second victim's apartment leads the team to suspect a recently-paroled felon named Marcus Gates. Castle and Beckett track down Gates after getting a tip from an old prison chum, Jerry Tyson. Gates works at a prop house which explains his prints were on the nametag. A video surveillance camera places Gates at his apartment close to the time of Kim Foster's murder. Beckett still believes Gates is their guy, she just doesn't know how he did it.
Castle relishes in the fact that Beckett seeks his theories. He suggests talking to Jerry Tyson, who is reluctant to cooperate. Gates was his protection in prison and thinks the guy will have him killed if he talks to the cops. Beckett offers to get Jerry an early release if he helps them. She also offers protection for his girlfriend, Donna Gallagher. Deal!
Jerry says Gates once attacked a woman named Sarah and worried that the cops were after him. Gates later busted a bottle over some guy's head to get himself locked up. In prison, he’d be able to lay low for awhile. Jerry's story checks out. Sarah Townsend was a victim of the Triple Killer. The police keep their end of the deal. Jerry Tyson is released. To keep her word, Beckett orders that Jerry's girlfriend be located and protected.
While holed up in a safe house, Jerry tells Ryan that Gates used a partner to help him get away with his murders; though he gets the feeling he never used the same guy twice. Castle takes a second look at the video surveillance footage and realizes that the man they thought was Gates is an imposter with sweaty hands, just like Paul McCardle’s. To make matters worse, Gates has disappeared.
Gates and McCardle were once in a foster home together. The team worries they may be going after Donna Gallagher to punish Jerry for squealing. Esposito is able to track down Gates as he's attacking Donna. Beckett needs leverage to get a confession. She sends Ryan and Castle to the hospital where Paul McCardle has been admitted for heart surgery. He recently got the money to fix a congenital heart problem. There are some "get well" flowers from his caring foster brother in his room. Beckett uses this info as leverage to break Gates.
Gates gets immunity for his foster brother, Paul. In exchange, he confesses to all of the Triple Killer murders. Ryan tells Jerry Tyson that Gates confessed and Donna is safe. But when Jerry doesn't ask about his girlfriend, Castle realizes that Donna was supposed to die. Jerry Tyson is the one who has been pulling the strings all along. He's the real Triple Killer!
BAM! Jerry knocks out Ryan and pulls his gun on Castle. He admits he paid for Paul McCardle's heart surgery in order to gets Gates to do his bidding. Castle realizes that Jerry is the one who had been hiding out in prison. Now that he's out, he's going to kill again and again and again. To keep suspicions at a minimum, Jerry allows a tied-up Castle to answer a call from Martha. Castle ends the call by saying "I love you." This leads Martha to believe something is terribly wrong.
Martha calls Beckett, who storms into the hotel room. Castle and Ryan are fine, but the Triple Killer is gone. Castle knows Jerry Tyson let him live to punish him. The anguish he feels now that he got away is almost too much to bear. It's a good thing he has a partner who knows exactly how he feels, and to hold his hand.