Castle and Beckett playfully ponder where they’d be if they hadn’t come into each other’s lives. She believes she’d be captain of her own precinct. He feels he’d be working on his second Pulitzer. Of course, if they’d never met, they wouldn’t be able to investigate the murder of a Swiss man named Mathias Gensler. A silver briefcase that was handcuffed to the guy has been cut off along with the victim’s wrist.
Castle believes a clue may be found at an abandoned coal plant. He finds an artifact inside the mystery case. BANG! BANG! BANG! Gunshots ring out. Castle and Beckett dive for cover, but they can’t escape the grenade that rolls their way. BOOM! An explosion knocks Castle off his feet. When he gets himself together, Beckett is nowhere to be found. Castle races back to the precinct where Esposito and Ryan don’t recognize him. He also doesn’t look familiar to their boss—Captain Kate Beckett.
When Castle offers up details of the latest murder case, he’s tossed into the interrogation room. He shocked to learn that six years ago he helped a Detective McNulty solve a mystery. The futures of many others then took a different path. For instance, Martha is a huge Broadway star. Lanie is pregnant and, judging by the look on Espo’s face, he’s not the father. A brunette Alexis lives in L.A. As for Castle, he’s still a playboy. He’s also writing drivel like his latest novel, Finite Laughter. It’s not even funny how bad it is.
After fending off the advances of a sultry, young woman, Castle does some research on parallel universes. He finds an image of the ancient Incan artifact he uncovered at the factory. He realizes that he needs to help Beckett find this item in order to get back to his real reality. Castle manages to convince his old team that he can be of some use on their case. He also dispenses some relationship advice to Ryan and Espo, who are not with the people they love in this reality. Neither is Beckett for that matter.
As Castle finds the artifact, a woman named Maria Sanchez holds a knife to his neck. Esposito and Ryan storm the room. CRASH! The artifact drops. It smashes into pieces. Castle realizes it must be a fake. He tries to help, but is booted from the precinct. Of course, he knows the place inside out, so he sneaks back in to watch the interrogation. Beckett catches him spying. She tosses him in a cell, but eventually sets him free. She admits that she’s a fan of his writing. She wonders why he ever killed off Derrek Storm.
The police come up with a partial photo of one of the shooters. Castle offers to buy Beckett a drink to chat about the investigation. He learns that she was never able to solve the case that made her want to be a cop. The shooting suspect is spotted. Beckett takes him down with a little help from her new partner in crime-solving. She feels the case is closed. Castle can’t believe it. The Beckett he knows would never take the easy way out.
Castle learns the reason Alexis moved to L.A. She couldn’t stand that he gave up after the negative reviews of his writing. She also feels that she is now living a futile existence. Castle sets her straight. Everything they do makes a difference, sometimes in indiscernible ways. Castle decides to take his own advice. He’s about to bring Beckett coffee when he’s abducted in broad daylight. The mastermind behind his kidnapping is the man who is building the mall at the coal factory location: Marcus Lark.
Lark has the ancient artifact in his possession. He believes Castle knows how it works. He orders his henchmen to shoot out his kneecaps until he gives up the answers he seeks. BANG! BANG! Beckett shoots the two gun-wielding bad guys. Castle’s lecture about how she was compromising herself hit home. That’s just not who Kate Beckett is in any reality.
BANG! BANG! Castle jumps into the line of fire when another bad guy starts shooting. He saves Beckett’s life. She wants to know why. That’s easy. He loves her. Castle loses consciousness while clutching the artifact. He wakes up to find Beckett staring into his eyes. She’s wearing an engagement ring. His engagement ring. Castle is back! His real reality is right where he left it. The case is closed and he now knows the reason why he hasn’t been sleeping. Castle asks Beckett to marry him, as in right now.
The stage is set for a wedding in the Hamptons. Castle stands with Alexis and Martha by his side. Beckett is walked down the aisle by her father. They exchange rings. They exchange vows. They share love, and now they share lives. As the few wedding guests celebrate this momentous event with some champagne, the bride and groom share their first dance together as Mr. and Mrs. Richard Castle. From this day forward, they will be each other’s partner in crime and in life. Always.