The episode begins with Castle preparing cappuccino for Kate. Kate came home late last night because she was on a case where some guy blew up an apartment downtown. She tells him that there was a man who was spotted fleeing the scene in his car, but there are no leads as of now. They flirt for a while. Kate receives a call from work and she is informed that they got a hit on the plates from the traffic camera footage. She leaves for work. Castle and Kate meet Ryan, who tells them that the car belonged to a man named, Archibald Fosse. Fosse killed his bookie with a car bomb, six years ago. He was supposed to do life, but his case got overturned by an appeal three months ago.
Esposito arrives with a guy named Diego, who was on his way to pay Fosse a visit. Diego tells the team that he connected with Fosse online. Fosse was supposed to sell him some baseball cards from the ‘30s. Kate and Castle along with the SWAT team, barge into Fosse’s apartment and learn that he fled. Kate spots him from the window and warns Ryan and Esposito. The duo manages to get Fosse. Fosse removes a device from his pocket and turns it on. Esposito calls Kate and tells her to get out from the apartment because he feels that Fosse has rigged that place with a bomb or some other explosive. On their way out, Kate steps on something and they hear a ‘click’ sound. Kate tells Castle that she is standing on a bomb.
Castle tries to downplay it by saying that it might not be a bomb. The guy from the bomb disposal unit arrives and confirms that Kate is standing on a bomb. There are no other bomb in the house. He tells her to remain still until the unit figures out how to diffuse the bomb. At the precinct, Gates tells Ryan and Esposito that Kate is standing on the bomb. She asks them about Fosse. Ryan tells her that Fosse has a vendetta against cops and those who sent him to prison. It looks like he wanted this to happen. Gates tells them to interrogate Fosse and ask him how to deactivate the bomb. The duo question Fosse. Fosse doesn’t tell them anything, instead he lawyers up. At the apartment, Castle tries to distract Kate by telling her that she instantly fell for him when she first saw him.
They end up talking about all their weird encounters. They hear the sound of a helicopter and Kate wonders if everything will be alright. Castle assures her that the bomb disposal team is very good at what they do. At the floor below them, the disposal team is trying to deactivate the bomb. An officer arrives and tells Captain Frank that the detonator has a disarm code, a combination of five numbers. He goes on to say that if they enter the wrong code even once, the bomb will go off. At the precinct, Fosse’s lawyer tells Ryan that Fosse is ready to give the code if he gets full amnesty, despite having killed a person the previous night. Later, Gates tells Ryan and Esposito that the DA is not ready to accept Fosse’s demands. Gates tells them that they cannot let a serial bomber run loose in order to save just one life.
Ryan tries to convince Fosse’s lawyer to talk to him. Esposito decides to go and talk to Fosse directly. When he reaches his cell, he finds Fosse on the ground, bleeding. Moments later, Fosse dies; he took his own life. Ryan tells Esposito and Gates that Fosse was suffering from Stage 4 brain cancer and that he was going to die anyways. Ryan also tells them about another bombing in Connecticut that took place last week; the guy who was killed, worked for the ADA who put Fosse in jail six years ago. They figure out that there could be more bombs out there. They also think that Fosse is trying to kill everyone who was responsible for sending him to prison. Gates tells the duo to make a list of all the people who were involved with the prosecution in the trial.
She tells them to get any information they can on the fragments from the earlier bombing. This could help the bomb squad identify the components, which would in turn, would help them diffuse the bomb Kate is standing on. At the apartment, Kate can feel the fatigue setting in. She thinks that she might not last long. She wants Castle to do something if she isn’t able to make it. Once again, Castle brings up the topic of who fell for whom at the first sight. They end up calling Esposito to resolve this issue. Esposito thinks that they are cracking up. Ryan arrives and tells him that the fragments from the bombs are all different and cannot help them determine the third bomb. It is up to the disposal team now. Frank and his team use fiber optics to get a view of the bomb. Frank learns that there is a timer on the bomb and tells Castle.
Castle sets a timer on his phone. Kate wants to know what is going on. Castle tries to lie, but eventually ends up telling her that there is a timer on the bomb and there are 30 minutes left. Castle tells Kate that, like always, he will try to identify Fosse’s story. It might lead them to their clue. He finds the baseball cards that he was going to sell. He wonders as to why a dying bomber would want to sell his baseball cards. Castle checks the cards and tells Kate that these were recently purchased. This means Fosse wanted Diego to do something. At the precinct, Ryan and Esposito question Diego, but they don’t get anything useful. Diego tells them that he works for the NYSED and he has never met Fosse, or anyone related to this case before. There are ten minutes remaining and Frank tells Castle that he has to leave the building.
Kate convinces him to leave. She tells him that she loves him. After Castle leaves, she calls her father and leaves a message saying that she loves him. Moments later, Castle returns with coffee. He tells her that he is not giving up on her. He has brought the detonator with him and is planning on diffusing the bomb. He wonders why Fosse planted the bomb next to the kitchen counter. He finds Fosse’s computer. He figures out that the bomb wasn’t meant for the cops. He feels that Fosse called Diego for some information. He concludes that Fosse was trying to find someone.
He calls Ryan, who tells him that they checked the list of people who were involved in the trial. They found out that Fosse’s girlfriend, Dorothy Novark’s, information is scrubbed clean from the systems. She is under witness protection because she feared that Fosse would come for her after his release. The previous victims were related to the girlfriend, and this proves that Fosse was desperately searching for Dorothy. Castle asks if the girlfriend has a child. Ryan tells him that she gave birth eight months after the trial. This means that Fosse was trying to search for his child because he wanted to see his kid before he died.
Since Diego worked for the State Education Department, he would be able to access the State’s school records. He tells Ryan to find the kid’s name because he is sure that is the code. There is less than a minute left. Ryan tells him that the kid’s name is William, but the code is a five digit number. Castle types in Billy and the bomb diffuses. Kate hugs Castle. They come out of the building and are about to kiss, when Gates arrives. Gates tells Kate to kiss Castle and thank the man who saved her life. Everyone is surprised to learn that Gates knows about them. The team leaves the couple alone, and they kiss. The episode ends.