Fifteen years ago, Frank Henson (James Carpinello) was convicted for the 1998 murder of a high school girl, Kimberly, and was sentenced to death. In the present time, a court refuses to grant Frank an emergency stay and he will be hanged in 72 hours.

Castle's (Nathan Fillion) daughter, Alexis (Molly Quinn), is working on an innocence review case for a law professor. She is present in the court when the judge denies the emergency stay. Alexis believes in the innocence of the man.

Alexis visits 12th precinct. Esposito tells her that nothing can be done. Ryan and Esposito tell Alexis that may be Castle will spot something that no one else did. She meets Castle and tells him that she needs help but that does not mean that things are okay. Alexis tells Castle that she is still mad at him.

Alexis believes in the innocence of the man because she spent time with him and know him. "I need you to come to Pennsylvania with me. I need you to help me prove it," she tells Castle.

Castle and Beckett had planned to look for wedding venues, but that plan has to be cancelled. Beckett says that she fully understands that and asks him to spend some time with Alexis. Castle thinks they are going on a road trip. Alexis reminds him that it is not a road trip and it is not at all about having fun. They have less than 72 hours to save a man from execution.

While driving to the prison, Castle tries to bond with his daughter by asking about Pi. Alexis is not eager to do that and talks about the convict and love of his life, Maggie. Maggie is Frank's high school sweet-heart and he had proposed to her two weeks before his arrest.

Maggie is there at the prison and she tells Castle that she had almost given-up till Alexis called. She sees hope, while Frank asks Castle he had saved anyone on a death row before. Castle says no, but Alexis chips in that her father is a genius.

Frank tells him what had happened at the time. The murdered girl had mentioned in her diary about having a crush on Frank and that projected him as some kind of a predator. Also, Kimberly had a stormy relationship with her boyfriend. According to Maggie, the girl had dumped him a week before her murder.

Castle calls up Beckett and is hyper. Castle tells Beckett that they (Frank, Maggie and Alexis) think he is some kind of a saviour, while he still does not know whether Frank is innocent or not. Castle fears that if he is unable to help, Alexis will never forgive him. Beckett tells him calm down, do his best and that she is there for him.

Kimberly was a bright, beautiful girl and had no enemy, as per the documents on her murder. While Castle and Alexis are talking about Frank and his motive, a cop interrupts their conversation. He says "you innocence review folks won't give-up, will you" and wants to know what they are fighting for. Alexis says "justice." The cop mentions that Frank had blood on his hands when they caught him. He adds that let Frank meet his maker as he had justice the whole time.

Frank had mentioned that he had entered Kimberly's house through the backdoor, which was open that night. Alexis points out that there were no fingerprints found on the knob of the door or on the door, which is strange because Frank's fingerprints were found at the rest of the place. The door was wiped clean. This revelation makes Castle believe that someone else was there in the house.

Lanie tells Alexis and Castle that they re-tested every swab taken from Kimberly's body. Everything that was listed in the Pennsylvania police department report came out in Lanie's report, except trace amounts of ammonium nitrate. The Pennsylvania lab never picked up ammonium nitrate that could be found in cold packs, fertilizers, explosives. Alexis points out that Lyle Gomez, Kimberly's boyfriend, used to work at a gardening store. She is convinced that he is the one who killed Kimberly.

Lyle does not want to talk to Castle and Alexis. Alexis thinks that he does not want to talk to Castle and that she can reach him. She uses Kimberly's diary to get herself invited inside Lyle's house. She tells him that Kimberly had mentioned in her diary that Lyle was kind, caring and protective about her.

Lyle tells that Kimberly had called him that night and she wanted him to pick her up from a place. Despite breaking-up with him, she had told him that he was the only person she could trust. Lyle mentions that Kimberly was upset but she did not tell him about what. Kimberly didn't let him in when they had come back, saying that she needed to be alone to make some decisions. Lyle tells Alexis that he then decided that he needed to get drunk and shoplifted but the shop-owner caught him and as a punishment he had to pick every litter piece in the parking lot. And when he had returned, he had seen Frank and the body of Kimberly on the stretcher.

Lyle believes that Frank killed Kimberly may be because he got jealous and did not like seeing her with other guys. Other guys used to drop by her house, according to Lyle. When Castle and Alexis want Frank to remember if there were other guys around her, Frank says that he is tired and it is over. He tells Alexis that "it's done."

Alexis is in tears hearing that, while Castle watches on. Later in the car, Castle tells Alexis that they are going to follow every possible lead but they are not going to go back to Frank without solid evidence. Ryan informs that there was trouble at the place that Kimberly visited. The farmhouse there was contaminated with toxic chemicals, which may explain the presence of ammonium nitrate. Castle figures out that someone was using the farmhouse to cook meth. The tyre-tracks outside the farm-house and found outside Kimberly's house matched, which make Castle and Alexis believe that Kimberly was going to turn the guys in for cooking meth and thus she was killed by one of them. Castle and Alexis at the same time say that they need to talk to Kimberly's mother.

Beckett is wondering that Alexis went to everyone for help, except her. She feels that when it comes to Castle and Alexis' relationship, she will be always on the outside. Lanie tells Beckett that there is nothing called perfect family. "It is up to you to make your own history."

Kimberly's mother tells Castle and Alexis that her daughter was only seeing Lyle. She also tells them that Kimberly was a tutor of Chemistry. Alexis tells Castle that two identical Chemistry books were found at Kimberly's place. The librarian tells them that both the books are still loaned out since 1998 and never returned to the library. One was issued in the name of Kimberly and another was loaned to John Henson, Frank's younger brother.

Ryan messages Castle and Alexis that John was once arrested for possession of a controlled substance, which is three months before the murder. John lives in Manhattan and thus Beckett questions him. He refuses to answer her questions when Beckett wants to know where he was that night, and walks off.

When Castle and Alexis tell Frank that John might be the murderer, Frank reminds them that he asked them to drop the case. Castle tells that Frank knows that it was John, and Maggie demands to know whether he knew the whole time. Frank feels responsible for John's action. Franks tells them the real version that he saw his brother beside Kimberly's dead body, crying when he had run to her house after hearing her scream. Frank had asked his brother to get out of the house and clean himself. "He doesn't remember anything from that night," Frank tells them. He does not want to take his brother's life away from him. John has a wife and two kids, and has built a life.

Castle has almost given-up as Frank is not willing to cooperate but then he finds out that the timing of the death was reported, incorrectly. It was John who had found the dead body of Kimberly and had screamed. Alexis says that both the brothers are innocent. Castle finds a dolphin pendant while watching the video footage of the investigation recording. Since it didn't come from Kimberley's bracelet, it must belong to the real murderer. Castle is waiting for Ryan's call related to cheques, which can lead to the name of the murderer, as Kimberly took cheque payments from her students.

It turns out the cop who had interrupted Castle and Alexis earlier was also one of the students that Kimberley had tutored. He is the one who killed Kimberley because she had found out about his meth cooking and he had joined her Chemistry class to learn precisely that. Frank is a free man.

At the end of the episode, Alexis meets Beckett to thank her and the two hug each other.