Beckett is surprised that Castle proposed to her on a swing set. She thought he was going to break up with her since he seemed so serious. She has a hard time saying yes because she must first tell him that she got the job in Washington. Castle says that he didn't propose just to keep her there with him. He wants to marry her no matter where she goes. She says yes and hopes they will be able to make it work.
Two months later, Beckett is in Washington and chasing down a criminal. The man runs into an alley and holds a woman hostage. She slides her weapon over to him and a flare goes off. Beckett uses this distraction to handcuff the man. She looks around for the hostage and finds her with a gun. The fake hostage shoots Beckett several times and she falls to the ground. Agent Rachel McCord rushes around the corner and over to Beckett. Rachel informs her that she has lost the launch codes to the Chechnyans. The whole scenario was just a test. Beckett failed to use her intuition to realize the hostage wasn't real. Hendricks invites Beckett out for drinks after work but she is busy with training and the caseload. She also has plans but not for long because there is a potential national security breach.
Castle returns home from his book tour and greets his mother Martha and his daughter Alexis. Alexis mentions that she wasn't alone on her trip and Castle isn't too happy to meet Alexis' new boyfriend, Pi. He's even less happy to find out from Martha that Pi is staying in her room. Martha leaves for a class and Castle talks to Alexis. She wants him to give Pi a chance and he says that they'll talk about it later. Beckett calls Castle and says she can't make it. He understands because he was the one to cancel last time.
Beckett is in her bathroom when she senses someone behind her. She pulls her gun out and turns around, only to find that it's Castle. He couldn't go another weekend without her. The next day Castle interrogates Beckett in an attempt to find out more about her case. She can't tell him anything but he suggests that if she does then he can help her solve it. However, she says no. As tempting as it sounds, she doesn't think the jail time would be worth it. Beckett leaves in a hurry and forgets one piece of paper on the floor. Castle finds it and studies it.
Castle calls Javier and asks about how a person might investigate a case with only one photo as evidence. He gives him the number from the transformer and they give him the exact location of the transformer and find that it is being investigated by the police already. Castle decides that the transformer was blown in order to cause a blackout.
Beckett finds that the blackout covered someone's theft of a module. Whoever took it is capable of shutting down a satellite network down and leaving a large portion of military defenses blind. The whole theft operation took less than five minutes but Beckett's boss has a hard time believing they could pull off the whole crime in that timeframe. Beckett suggests that maybe they escaped underneath the building in the sewers. They decide to investigate nearby sewer exits and decide on the golf club drain. At the golf club, Beckett and Rachel ask a security guard about the blackout. He says he answered this same question for another man and he points over at Castle. Rachel is mad that he is there but Beckett doesn't understand how he found anything out. They confront him and he says he is just golfing. Rachel gives him a warning and tells him that she doesn't want to see him around again. Beckett tells him that she wants to see him too but he can't be a part of the investigation. Castle tells her that he has some information but it's nothing helpful.
Back at the office, Beckett thanks Rachel for not saying anything about Castle. The two of them view the golf course security video and find a suspicious man. They break into his house but don't find him. He did leave C4, wire cutters and rope pulleys that pin the module theft on him. Castle calls Becket while she is working and asks how the case is going. He promises that he won't say anything. After he hangs up, a man holds a gun to Castle's back and directs him to his car. It is the same man from the gold course surveillance video. The man asks if the feds said anything about Valkyrie. He soon passes out and Castle needs to take over the steering. Even with his help they still manage to drive into a bus stop.
Rachel and Beckett are shocked to find that Castle is once again involved and Rachel arrests him. Castle is interrogated and tries to explain that he was abducted. It sounds suspicious that Castle was in the car when the man died.
Hendricks finds the girlfriend of Bronson, the dead man. Her car and phone were found abandoned and the chief wants her found. Beckett is told to stay behind and that she may be in trouble because of his involvement. Beckett promises him that they will make things work. After this case she will have a few days off to spend with him.
Castle returns to his home and is frustrated to be stuck alone with Pi. Castle asks how long Pi is planning on staying at his house. Pi says he usually lives in Amsterdam but he has lost his passport. Pi offers to talk about what happened on his trip to Washington. Castle says talking about it sounds great but leaves to find someone else to talk about it with.
Rachel asks if Castle made it home alright and if Beckett wants to talk about it. She says no and Rachel is glad. They find Bronson's girlfriend Jeanette but lose track of her. They track her path and it looks like she is headed to Union Station to make a run for it.
Beckett meets with Javier and Kevin and tells them about his Washington trip and how he was abducted. He tells them about the Valkyrie thing he was rambling about. They ask if he was a veteran and Javier says that he had a friend who knows of a ghost base during the Gulf War that sounds familiar. If this is what Beckett is after, he warns that it is way too big of a problem for her to take on.
Beckett and Rachel search Union Station for Jeanette. They think that maybe Jeanette isn't running but meeting with her buyer. They follow Jeanette and see her open a locker. They arrest her and find a briefcase in her locker that contains the module they are searching for. They bring her in for questioning. Jeanette says that Jack is being set up. Jack said that they needed to go on the run until they figure out what is happening. They ask her about Valkyrie but she doesn't know anything.
Beckett believes that Jack Bronson was being set up because someone with his level of training wouldn't slip up like this. Rachel and Beckett can't figure out why someone would steal it and then give it up so easy. Beckett believes that it may have been a distraction. The building that the module was from has a classified floor 8.
Back home Castle tells his daughter that while he is still researching Beckett's case, no one will ever know. Just then two federal agents come in and say they need to take him with them. Castle is once again in the interrogation room but this time Beckett explains what is going on. The classified floor has a chemical agent that was deemed too dangerous to be used. The amount of the agent stolen is enough to kill thousands of people. The toxin is what was used to kill Bronson. It was in the car's ventilation system. Castle asks if he was poisoned and how bad it was. Beckett says that based on the amount in his bloodstream, Castle has one day left to live.