It's coffee in bed for Beckett as she was out late investigation an apartment explosion. Castle tries to seduce his partner away from following an early morning lead. She's powerless against his charms. Well, that's what he believes. As for the case, the suspect is Archibald Fosse. He blew up his bookie six years ago and his conviction was recently overturned. Fosse was being visited by Diego Jimenez to follow up on an online baseball card deal. He bolts from his apartment when the police show. Ryan and Esposito catch him as he activates a handheld device. Back in his apartment, Beckett senses something under her feet. She thinks she's just stepped on a bomb.
Castle believes everything is fine. Beckett probably stepped on a loose floorboard. Or it could be a trigger plate. Captain Mahoney of the bomb squad says the latter is true. Beckett is standing on a sensor plate that's wired to a detonator. One wrong move and the entire building will blow. All non-essential personnel are order to evacuate. This includes Castle. Yeah, that's not going to happen. Castle is going to be Beckett's bomb buddy. The only way he'll leave is if she admits that she had feelings for him when they first met. Surprisingly, Beckett does admit she had one feeling for him back in the day. She couldn't stand him.
Castle and Beckett recall the early days of their partnership. He was a total playboy and she's had a dizzying array of hairstyles over the years. There was also banter, lots and lots of banter. Okay, some things never change. The trip down memory lane keeps Beckett's mind off her dire situation. The only way to disarm the bomb is to punch in a code. There are a hundred thousand possible options. Only Fosse knows the code. His motive appears to be revenge against people who wronged him during his trial. He may also have a vendetta against cops. Fosse's lawyer says he's client wants full amnesty before he tells them how to save Beckett.
Esposito is about to take drastic measures to get Fosse to talk when he finds the bomber bleeding out in his cell. The suspect stabbed himself in the neck with a pen. Turns out Fosse had brain cancer. He was dying and figured to take a few of his enemies with him. That now includes Beckett, who is starting to feel fatigue. Castle embarks on another effort to take her mind off the situation. More memories are recalled about all the times others told Beckett that she was crazy about her partner. As for Castle, he's lost in thought about all the revealing outfits Beckett has worn over the years just to get his attention. She claims it was all in the line of duty.
Castle refuses to admit that he was hanging out with Beckett all those years because he was into her. He just loved playing cop. The debate about who liked who first rages on. Esposito and Ryan believe the bomb buddies are starting to crack under the pressure. Things go from bad to worse when the bomb squad learns there's a timer on the device. There's less than 30 minutes before it blows. Castle promises they'll get through this. They've been in scary spots like this before. Of course, the most terrifying moment was when they were nearly caught in bed by Martha after their first night together. Talk about close calls.
Castle believes the suspect's story will lead them to a solution. It's worked for them before. They realize that Fosse may have been luring the baseball card collector to his apartment for a reason. This info may be too little too late. There are only 10 minutes before the bomb blows. Beckett made Castle promise to leave her alone when the time came. The two of them exchange a heartfelt "I love you" before saying goodbye. Beckett leaves a voicemail message for her dad to say she loves him, too. She recalls so many emotional memories as she stands alone with closed eyes. Then she's not alone anymore. Castle is back. He brought coffee.
Castle promised that he'd leave, but he never said he wouldn't come back. He wonders why the bomb sensor was placed in such an odd location. The plan was to get Diego the baseball card guy to stand there so he'd look up info on the nearby computer. Fosse had a son who had been hidden from him. He was desperate to find the kid. Diego could help. Castle believes his son's name is the code to deactivate the bomb. They have two minutes to find his name. One minute. Time is almost up. The boy's name is William. The code needs to be five letters. Billy. Castle punches in the name. Time's up. Castle was right. Beckett is safe.
Cheers erupt back at the precinct as Castle informs everyone that his partner is off the bomb. Beckett thanks him for staying with her. Castle's response: Always. Ryan and Esposito rush up to the scene to give hugs to their colleagues, their friends. Captain Gates is there, too. She thinks Beckett should give him a kiss. She knows all about the two of them. The woman's not an idiot. She says Castle deserves a smooch after what he did. Beckett agrees. She wonders about all their past kisses. Which one was the best? Castle's asks her thoughts on that perplexing question. Beckett answers, "I think we're just getting started." And then they kiss again.