Castle doesn’t normally make me cry (it’s a light-hearted “dramady” starring Nathan Fillion, after all!), but last night it did. As always, spoilers abound--bigger and more intense spoilers than ever before.

Remember Hal Lockwood? He was the professional killer that pulled the trigger on Beckett’s mother all those years ago. Beckett put him in jail earlier this season, but Beckett promised Lockwood that she would interview him every week and continue to ask him who hired him to kill her mother. “Knockout” picks up as Beckett makes her weekly journey to the prison to interview Lockwood. Once she arrives, a guard (Riker, remember him, he’ll be important later) tells her that Lockwood has been transferred from administrative segregation to general population.

Beckett tells Riker that Lockwood will kill another prisoner in general population (Gary McAllister) unless the guards get to him first. McAllister, if you remember, was a dirty cop connected to Beckett’s mother’s case. Lockwood has been systematically hunting these cops down, but the fact that McAllister was in prison saved his life. For now. The cops arrive at McAllister’s cell too late--Lockwood has killed him. Ryan and Esposito do some digging--the form to transfer Lockwood to general population carried the forged signature of the warden. The only people with access are correctional officers. They run the financial records on everyone who works in the prison.

This murder allows Lockwood to be taken out of the prison for an arraignment. Castle and Beckett go to see him to rattle his cage--and the cage of whoever paid Lockwood. At the courthouse, imitation cops throw flash-bombs into the courtroom. Lockwood escapes. Beckett chases, but Lockwood climbs into a helicopter and flies off into the night. Back at the precinct, Ryan discovers something interesting in Lockwood’s phone records. Prisoners have to make collect calls, and every week Lockwood called one number. The person on the other end refused to accept the charges for weeks, until one day they answer. It’s a burner cell, but they’re speaking in code. The message: continue the mission. Lockwood didn’t just want to kill McAllister, but the last renegade cop involved in Beckett’s mother’s case. But who is this third mystery cop?

Montgomery puts a detail on Beckett--just because Lockwood didn’t kill Beckett in the courtroom when he had the chance doesn’t mean she’s not in danger. Montgomery also gives a chilling order to Ryan and Esposito: kill Lockwood. He doesn’t deserve to see the inside of another courtroom. Ryan and Esposito agree, but Castle overhears and looks troubled. Jersey state police call--they found the stolen helicopter in a hanger. The inside has been cleaned with bleach, leaving no physical evidence.

Back at Castle’s apartment, Beckett’s father, Jim shows up. He’s worried about Beckett, understandably, and he tells Castle that he doesn’t want Beckett to lose her life fighting this. He wants Castle to convince her that “her life is worth more than her mother’s death.” The next morning, in his own Castle way, he tries to do this, but Castle is interrupted by Ryan and Esposito. When they ran the prison employees’ financial statements, they discovered an unusually large wire transfer (50 grand!) to Officer Riker, Beckett’s CO friend from the beginning. Guess who didn’t show up to work today? Riker. But not because he ran, as Beckett and company discover when they burst into his apartment, it’s because Riker was shot in the head.

Beckett and Castle go to the prison to interrogate his friends and co-workers. They receive uniform answers: good guy, upstanding officer, even the prisoners liked him. No one knew he was having financial trouble, so how did the killers know he would be open to blackmail? Beckett has the answer, of course. The bad guys ran Riker’s financials, just like Ryan and Esposito did. That means the third cop isn’t Lockwood’s target, but the one holding his leash. Beckett tells Ryan and Esposito to go through records again. When Esposito protests (they already went through the records, twice), Beckett snaps. I don’t think I’ve ever heard Beckett raise her voice to the members of her own team, but she did in this episode. She shouts that no one wants Lockwood (and the third cop) as badly as she does, so they will check it again. Everyone looks scared.

Esposito walks in carrying a six pack of beer. Montgomery doesn’t even hesitate to give them permission to drink it. A name on one of the records catches Castle’s eye. Napolitano, along with Raglan, are listed as the arresting  officers on a report. Ryan and Esposito dismiss Napolitano as the third cop because he was at his daughter’s wedding the night the three cops murdered a gangster. Besides, Napolitano died in the early 90s, so why does it matter? It matters because Castle is a) a genius and b) can recognize the different ribbons on typewriters of different eras. Someone used white-out to cover up the original officer’s name and used a newer typewriter to cover it up. Ryan and Esposito investigate who was in charge of the records room between the years of the gangster’s murder and when the police station went digital (roughly a decade), to see if they remember anything.

While Ryan and Esposito dig deeper, Castle takes the time to share a moment with Montgomery. They’re both worried about Beckett, that’s clear. Castle wants Montgomery to take Beckett off the case. Montgomery responds with a story of the first time he met Beckett. He went down to the archives and found Beckett--still a patrol cop--pouring over a case file. She wasn’t authorized to view it, of course, and Montgomery asked her why she was down there. She told him it was her mother’s murder and she found things that didn’t add up. Montgomery saw something in Beckett’s eyes--a refusal to retreat, to give up, to back down. Castle points out that Beckett’s tenacity will get her killed, but Montgomery points out that Castle is the only person who will be able to get Beckett to back down.

Cut to Beckett’s apartment, night. Castle knocks, Beckett lets him in. She’s holding a gun, looking a little frazzled. Castle confronts her with the knowledge that everyone knows and no one has talked about: everyone associated with the case is dead, and Beckett is probably next. Castle asks Beckett, plain and simple, to walk away. He calls her Kate, which happens so irregularly it makes you sit up and listen. He tells her to think of the people who love her: her father, Josh... Beckett counters with “what about you, Rick?” Castle tells Beckett that he cares about her, of course he cares about her, because they’re friends. Beckett asks the question both of them have been thinking for years: “is that what we are?”

In a moment better than their banter, better than their kiss, Castle tells Beckett that he doesn’t know what they are. They kiss, they almost freeze to death, but they don’t talk about it. The argument escalates as Castle says the unforgivable: he tells Beckett that it isn’t about solving her mother’s case, but using it as a place to hide. He tells her that she’s forgotten who she is outside of the mystery of her mother and that she can’t bear to lose the identity she’s formed. Beckett kicks him out of her apartment.

Beckett goes to Montgomery and tells him she wants Castle gone. Montgomery agrees and for a moment Beckett hesitates. She asks about the mayor, using the excuse Montgomery has always used as to why Castle sticks around. Montgomery admits that he let Castle follow Beckett for so long because she seemed like she was having fun for the first time ever. He tells her that, as homicide cops, they speak for the dead. They owe the dead their voices, but not their whole lives. That being said, Montgomery tells Beckett that he will back her up no matter what.

Ryan and Esposito have sequestered themselves in the stacks of records, going through paperwork. They discover that Sgt. Mike Yanavich ran the records room during the time the records were altered and that he graduated with Raglan and McAllister. He currently owns a cop bar in Brooklyn.

That night, Montgomery is in his house when he receives a surprise guest. Lockwood, holding a teddy bear more menacingly than I have ever known someone to hold a teddy bear, stands in the dark. He threatens Montgomery’s family and references an agreement he made: Beckett would stay alive if Montgomery could keep her on the leash. Montgomery is forced to choose between Beckett and his family. Lockwood tells Montgomery that if he and Raglan and McAllister “hadn’t sinned so spectacularly then God wouldn’t have sent a punishment like Lockwood.” This can only mean one thing: the third cop is Montgomery. What. I’m going to give you guys a second to let that sink in. Montgomery. A rogue cop. This is rather upsetting!

The next morning Montgomery says goodbye to his family, puts secret documents from a safe into an envelope, stamps it and loads a gun full of bullets. I know Montgomery would never choose Beckett over his own family, but I still hope he can find a way to warn her! Back at the precinct, Ryan and Esposito go to the cop bar Yanavich owns. Montgomery calls Beckett and asks her to go to the hanger in Jersey where they found the stolen helicopter, the place Lockwood told him to lure Beckett. Beckett, trusting as always, follows Montgomery’s orders.

At the cop bar, Ryan and Esposito hit a dead end. Yanavich didn’t really know Raglan and McAllister, but he remembers a rookie cop that used to hang around. Yanavich has an old picture... of Roy Montgomery. Ryan and Esposito run out of the bar. Ryan tells Esposito that Montgomery has to be the third cop. In the third saddest moment of the entire episode (the first and second haven’t even happened yet), Ryan and Esposito brawl in the alley. You can almost hear the sound of their hearts breaking as they realize their mentor betrayed them.

In the hanger, Beckett approaches Montgomery. Suddenly her phone chirps--it’s a text from Ryan (or Esposito) naming Montgomery as the third cop. She confronts Montgomery, who admits to the whole thing. He accidentally shot the mobster all those years ago. He didn’t hire Lockwood to kill Beckett’s mother, but he won’t tell her who did. He used the incident to become the best cop he could--as penance for his crimes. He tells Beckett that he didn’t lure Beckett to the hanger to kill her, but instead to lure Lockwood and his associates, to kill them instead. Castle emerges from the shadows to take Beckett away.

In the single most heartbreaking moment of the entire episode, Castle handily picks Beckett up and drags her, kicking, screaming and sobbing, out of the hanger where they wait, silently, as Montgomery first guns down Lockwood’s associates then Lockwood himself. But it’s too late for Montgomery--Lockwood shoots him and Montgomery bleeds out on the hanger floor.

The next scene is Montgomery’s funeral. Beckett tells Castle, Ryan and Esposito that no one will ever know that Montgomery died that way. She wants everyone to remember him as a hero--as a good cop. As Beckett delivers the eulogy, as the audience feels safe, comfortable and sad, Beckett is shot by a sniper in the cemetery. Castle runs to her side and holds her in his arms, staunching the blood with his own hands. He tells Beckett to hold on. He calls her Kate. He tells her he loves her. Beckett’s eyes close, and the season ends.