Picking up where Tick, Tick, Tick... left off, Beckett's apartment has been blown away. Castle breaks down the door, frantically calling her name. Amazingly, Beckett is alive! She survived the blast by jumping into her tub. And she happened to be naked. With her towels and bath robes in smolders, Castle covers her body with his coat (don't worry, he didn't look) and they escape the fiery ruins.
The team realizes the killer must have remained in Ben Conrad's apartment when the FBI showed. Beckett finds a false wall in the closet. Inside are the remains of the real killer's disguise and duct tape residue that indicates Conrad was being held hostage. Beckett gets a call. It's the killer. He's disappointed Nikki Heat survived. This was supposed to be over. Now he'll have to kill again.
Castle points out that the killer must've known Ben Conrad to be able to pose as him. After finding out that Conrad was a Knicks fan, Beckett and Castle discover that he made a friend at a local sports bar. The bartender remembers calling the guy a cab one night. Using FBI technology, Agent Shaw zeroes in on possible suspects in the area. One of the matches, Chris Doherty, was murdered six years ago. The killer assumed his identity.
Beckett's team and the FBI storm the suspect's apartment to find a shrine to Beckett/Nikki Heat. Castle skims the pages of a manuscript and realizes the killer was writing a book based on his murders. The FBI sets up a stakeout on the apartment. Defying Shaw's orders, Beckett leaves the surveillance van when she spots the killer close to her position. She chases him down, but loses him on the subway.
The feds use facial recognition software to get an ID on our killer, whose name is Scott Dunn. Before Beckett can mobilize her team, Shaw takes her off the case. Needless to say, Beckett is upset. She can't work and can't go home. Well, she can't go to her home. Castle, however, offers her his. He says, "It's a secured building with an extra bedroom and people who care about you."
Back at Castle's place, he and Beckett chat about the case in their usual bantering way. Beckett asks Castle if he's sorry he wrote Heat Wave. Castle says, "The way I look at it now, if it wasn't for Nikki Heat, this guy would have just gone on killing because he wouldn't have met anyone smart enough to catch him. I'm speaking, of course, about Special Agent Shaw." Case or no case, some things never change.
Speaking of Agent Shaw...she's in trouble. Dunn hides out in her car and kidnaps her at gunpoint. He wants Beckett to meet him alone or Shaw dies. When Beckett asks for proof that Shaw is still alive, Dunn emails her a video of him holding her captive. Looks like Beckett's back on the case.
Using clues in the video, they are able to triangulate the location to an abandoned building. Castle thinks this is too easy. Once the FBI agents are gone, Beckett asks Castle what's bugging him. He says this isn't the way he'd write the scene. He'd make the FBI swarm the building and then blow it up. And he'd be watching it all from a building nearby. Beckett hands Castle her backup pistol and tells him to follow her. He's her only backup.
In a neighboring warehouse, Beckett spots Shaw bound to a chair with tape across her mouth. Beckett confronts Dunn, bluffing that the FBI agents are not in the other building. That means Nikki Heat won. Dunn panics and throws the detonator-phone at Beckett, fleeing deeper into the warehouse. Castle tries to untie Shaw, who urges him to go help his partner.
Beckett gives chase, but Dunn gets the drop on her. Castle arrives to find Dunn standing over Beckett, a gun pointed at her head. BANG! Castle fires, hitting Dunn's gun and knocking it out of his hand. Dunn dives for the firearm, but Shaw steps on his hand, a gun aimed at his head. Beckett cuffs Dunn, who says, "This isn't over, Heat." Our favorite homicide corrects him by saying, "It's not Heat. It's Beckett." Love it!
With the killer apprehended, the FBI takes its expensive toys and bugs out of the precinct. Shaw and Beckett say goodbye, each having grown to respect the other. Regarding her partnership with Castle, Shaw says, "He cares about you, Kate. You may not see it. You may not be ready to, but he does." Hey, we didn't need an ace FBI profiler to tell us that one.