Beckett fields a call from a man who says he's a "big fan" and claims to have committed a murder. The body of Alex Peterman, a personal injury attorney, is found at Grand Central Station. The five bullets dug out of his body are etched with letters that spell "Nikki." Believe it or not, this murder has been dedicated to Nikki Heat.
The killer calls again, addressing Beckett as Castle's favorite fictional character. He's claimed another victim. Four slugs pulled from a woman found dead at the Central Park Carousel are marked with letters that spell "will." It's a message: "Nikki will..." FBI Agent Jordan Shaw (guest star Dana Delany) and her entourage of Special Agents swarm the scene. This is their case now.
The second victim is Michelle Lewis, a dog walker. The FBI builds a high-tech war room at the precinct where various smart boards and computers are set up. Castle loves all the cool gadgets. He's also a big fan of Agent Shaw. Beckett is seething. We're not sure if it's because Shaw is invading her territory with the case, or if it's because she's invading her territory regarding Castle. What do you think?
A print on Michelle Lewis's purse matches the pinky of a paroled man named Donald Salt. His partial print was also found at Grand Central Station. Coincidence? No way. Shaw forces Castle to stay in the car during the arrest. But when Salt spots the feds and bolts, Castle takes him down with a Taser he'd been playing with in Shaw's vehicle.
As Beckett cuffs Salt, she notices his hand is bandaged and his pinky finger is missing. Salt sold his finger online to the killer, who used the displaced digit to trick the cops. Beckett notices a series of numbers written on Salt's bandage. The killer left a code.
Castle cracks the code as page numbers and words in Heat Wave! They decipher the message, which reads: "I will kill someone else before midnight tonight unless you stop me." The killer calls again to tell Nikki Heat that he's disappointed she hasn't caught him. Now someone else has to die. After tracing the call to a parking garage, the team finds a woman's shoe, strands of hair, four shell casings and lots of blood. But where's the body?
Worried about Beckett, Castle spends the night on her couch. No, really. He never left the sofa. The next morning, he opens her front door to find the missing dead woman falling at his feet. The bullets in this body have letters that spell "burn," which means the whole message reads, "Nikki will burn." Not good.
The third victim is Sandra Keller, a taxidermist who argued with a customer matching the killer's sketch. The customer's dog was killed when Michelle Lewis, the 2nd victim, lost control of the Rottweiler she was walking. The customer had his dog stuffed, but couldn't pay for it. Two nights later there was a break-in. The dog and a bunch of formaldehyde went missing. The lawyer, Alex Peterman, is also connected, having declined a lawsuit over the dead dog. The client? Benjamin Conrad, a man whose DMV photo matches the sketch of the killer.
As the FBI team moves in to arrest Conrad, Beckett gets another call from the killer and sees him through a window, a gun in his left hand and a cell phone in his right. He says that someone has to die and if it can't be Nikki Heat... BANG! Shaw and her team storm the apartment to find Conrad dead from a gunshot to the head.
Conrad was using the formaldehyde from the taxidermy theft to make a bomb to blow up Beckett and others at the precinct. Later, Castle is going through crime scene photos when he has a realization: In the pictures, Conrad lays dead with the gun in his right hand, but the killer they saw in the window was left-handed. Ben Conrad didn't kill himself; he was murdered by the man they saw in the window. The killer wanted them to think it was over so he could... make Nikki Heat burn!
Stepping out of the shower, Beckett picks up her ringing cell to hear Castle frantically yelling that Ben Conrad wasn't the killer. She has only moments for this to register when another cell phone rings and she hears an automated voice say, "Goodbye, Nikki." BOOM! Beckett's apartment explodes in an enormous fireball as Castle watches helplessly from the street.
TO BE CONTINUED...though not soon enough.