Alex Peterman, a personal attorney, is found shot to death at Grand Central Terminal.
The murderer is calling out Nikki Heat, which is the main character from one of Rick Castle's fiction novels and based on Beckett.
This time, the body is found in the Central Park carousel, and the letters on the bullets left behind spell "will".
The FBI waste no time in making themselves at home in the precinct, quickly moving in their equipment required for the "war room".
The team manages to get a print off of Lewis' purse, which matches a man by the name of Donald Salt (Andrew Rothenberg), who is on parole.
Salt tries to run from the police, making a mistake by going straight past the car where Castle is hidden inside.
Stepping out quickly, the author stuns the fugitive with a high voltage taser gun, found in special Agent Shaw's SUV.
In interrogation, Beckett realizes that Salt is wearing a bandage on one of his hands and he informs her that he sold his pinky finger online to the real killer.
While on the phone, Castle hears the "Goodbye, Nikki" recording seconds before arriving at her building and then witnessing Beckett's apartment explode.
He takes cover from the blast and flying glass, looking up in horror to see the apartment on fire.
However, Castle broke that record with the second episode in the two-parter with Boom!, which attracted 14.5 million viewers.
Jonah Krakow of IGN, while noting that it was unlikely Beckett would be killed off, praised the creepier tone of the story, writing, "Normally, Castle is a fun, cute murder mystery show that doesn't take itself too seriously, but it was nice to see things get scarier in this two-parter.
"[4] Entertainment Weekly's Mandi Bierly said, "Castle has a lot to live up to after the way ABC has been promoting the two-part 'event'... Last night, Part 1 did.