A gloved hand dials 911 but no words are spoken. The mystery caller places the phone near a running shower where a dead man is sprawled in his tub. The victim is Ted Rollins-a 20-year-old record store worker was shot twice in the chest. Castle finds a laptop hidden in the floorboards. The victim's family says Ted was a computer genius who once stole $1.4 million in an identity theft scheme. All signs indicate he was hacking again. He was connecting to a company called Universal Banking Solutions. The Manhattan office is run by a gentleman named Anderson Cross, who happens to look a lot like Castle's father.
Castle first got to know his dad as Jackson Hunt in "Hunt." As Anderson Cross, he secretly tells his son to not blow his cover. Lives are at stake. Anderson Cross says his company had been hit with an attempted hacking just last night. A short time later, Castle meets his dad at a park where he reveals the details of the case. One tidbit involves the fact that Rollins carried an Iranian coin in his sock before he was killed. It was of very little value though. It's determined that the victim was killed underneath the roller coaster at Coney Island. Video footage shows Anderson Cross was at the scene of the crime. Beckett proclaims, "He's our killer."
Every sign of Universal Banking Solutions is wiped from the Internet. The office has been cleared out, too. Beckett realizes that she's seen Cross before during Alexis's kidnapping case. She says that this guy has been disowned by the CIA for an unauthorized assassination. He broke out of prison after killing three guards. The rumor is he's currently working as a hitman. Castle is at a loss. He confides in Martha, who advises him to lean on the people he trusts like Beckett. Castle is unsure of what to do next especially once his dad shows up at his front door with a gunshot wound to the gut.
Castle and Martha work to treat the gunshot wound. Jackson Hunt says he works outside the system. To do so, he needs an effective cover. Being labeled as a hired assassin being hunted by the U.S. certainly qualifies. Castle introduces Beckett to her future father-in-law, aka her prime suspect. Jackson reveals there was a massive breach at the CIA revealing the identities of deep cover operatives. It was inside job at the highest level. That's why they called in Jackson. He was ordered to find Gemini, which is the codename for the mole. This is the person who killed Jackson's hacker helper, Ted Rollins.
The Iranian coin was a clue as to who the buyer is for the stolen information. Jackson was shot while tracking down a lead. Gemini ambushed him. He had to drive into the East River to get away. Beckett doesn't believe him until news reports confirm Jackson's story. Ryan and Esposito find a flurry of gunshot markings at the apartment Ted Rollins used for hacking purposes. The hard drive from his computer is missing. That's because Jackson has it. He needs to borrow Castle's laptop.
Ted Rollins flagged an email from someone named Izad Larjani, who is part of Iran's CIA. The list of stolen identities is scheduled to be sent over a secured Wi-Fi connection at the Central Library. That means Gemini will be there, too. Jackson has to stop it, but he's in no condition to do so. That's why Beckett says she and Castle will step in. They need to login first once the network goes online in order to lock out the Iranians from downloading the list. Jackson gives his son a hug to thank him for what he's doing. Castle and Beckett head off to the library while Martha and Jackson stay behind to briefly reminisce about their one night together.
The network goes online at the Central Library. Castle begins downloading the files while Beckett searches for Gemini. She heads into the book stacks. She's close. Castle tells her they don't have much time left as he declines multiple incoming calls from his mother. Martha is trying to contact her son to let him know that his father has skipped out of the loft. Beckett learns that Gemini set up some kind of relay system. He could be anywhere. In truth, Gemini is inside the library holding a gun on Castle.Castle recognizes the gunman as a homeless guy he saw at the park when he met his father. Gemini was using this meeting to draw Jackson out into the open. He is about kill Castle until Jackson shoots him dead. That hug between father and son was actually Jackson's way of slipping a GPS locator into Castle's pocket. By the time Beckett arrives, Jackson and Gemini's body are gone. The murder weapon used to kill Ted Rollins is the only thing left behind. It's registered to Tony Blaine, a researcher at a government think tank. Case closed... Sorta.
Castle realizes that his father moved the original victim's body into the city so the case would be in Beckett's jurisdiction. This way he could work his son for information. Beckett suggests that maybe this was his way of getting to see him again. Castle keeps making the mistake that his father is his family. He's not. Beckett is. That's why he wants to marry her in September. As for Martha, she'll always be grateful to Jackson for giving her the greatest gift of her life. Mother and son gaze out the window wondering if they'll ever see Jackson again. They have no idea that he's on the street below looking up at them as well.