Illegal Czech immigrant Eliska Sokel's corpse is found hidden in a sewer, two days after her death. Her also-illegal ex, Teodor Hajek, explains she left him after their son died from a congenital disease. She had worked on the concierge staff of a luxury residence but was fired after Dr. Cameron Talbot's wife questioned the appropriateness of her treating their son Zane to a lollipop. Castle wonders how Beckett will take his dedication of a book starring her alter-ego, and his agent Haas's offer of a contract to write 3 British spy novels. They still solve the case after unearthing an old, tragic secret.
Castle's book agent, Paula Haas, approaches him with a career changing opportunity, but taking it would mean the end of his relationship with Beckett. Will the murder of an unidentified woman found dead in a manhole be their last case together?
- Paula Haas, Castle's agent, shows up to tell him that he is at the top of a list to write books on a certain British secret spy.
Elsewhere, Beckett and company investigate a murder scene where the victim, a Caucasian female, is found in a manhole. From the candy wrapper found on her clothing, a late arriving Castle deduces she is a Czech immigrant. At the shop where the candy was sold, Esposito and Ryan find out where the immigrant, Eliska Sokol, lived.
When they arrive at the apartment and start searching, Castle finds a picture of a woman and a child with the woman's face scratched out.
Martha makes Castle doubt his contribution to solving cases and when he learns Beckett has already located the woman in the picture, his fear that Beckett doesn't need him deepens.
The hotel manager says that Eliska used to work there but she was fired when Mrs. Melissa Talbot registered a complaint against her. Mrs. Talbot tells them that Eliska would often be over-familiar with her son Zane and give him a lollipop.
Eliska's ex-husband, Teodor Hajek, tells Castle and Beckett that she left him after their three-year-old son died. With seemingly no leads, they check her apartment once again and find a tenant who tells them Eliska had an argument with a man who resembles Dr. Cameron Talbot, Melissa's husband.
Castle and Beckett confront Talbot at his office. He confesses that he was having an affair with Eliska and, when she got too close to his family, he got her fired. He is alibied out by his head nurse, who says he was at the hospital until late in the night on the day of the murder.
At the gala opening for 'Heat Wave', Castle and Beckett argue over about whether to accept the offer to write the British spy books and leave Nikki Heat (and Beckett).
Castle goes to the precinct to pick up his sun-glasses and, as he is about to leave for the final time , he happens to overhear that Eliska's landlord has asked permission to rent out the apartment even though the rent is paid up until Friday. Since the rent is collected weekly and Eliska was killed the previous Thursday, Castle reasons that someone else must have paid this week's rent. Also, when the landlord states a blonde woman is collecting all of Eliska's mail, they conclude it must be Melissa Talbot.
But it turns out to be Dr. Talbot's head nurse, who was instructed by Dr. Talbot to retrieve a check he had sent to Eliska and thereby remove any connection between them.
Esposito and Ryan come back from talking with a woman who filed a prior complaint against Eliska for hanging around her child. The reason she notice Eliska is she remembered her when they were both in the hospital having a baby on the same day. Castle starts to get an idea and asks Esposito and Ryan to check the birthday of Melissa's child and that they will probably found that he was also born on the same day and at the same hospital, Bryant General, where Dr. Talbot works.
They call in the Talbots and tell them Eliska was going to do a DNA test on swabs she had used on Zane while giving him the candy.
Dr. Talbot breaks down and confesses that since Melissa's boy was was diagnosed with Niemann-Pick disease, which is incurable, he had switched the name tags on the babies cribs so that Melissa took home Eliska's healthy boy. When Eliska got too close to the truth, he killed her.
Melissa, who will keep custody of Zane, invites Teodor to her home to see Zane and make him a part of Zane's life.
As Castle and Beckett are leaving, both of their phones ring at the same time. Castle's call if from Paula, his book agent, informing him that 'Heat Wave' is flying off the shelves and they want him to write three more books for a lot more money than the British spy books. Beckett's call if from Captain Montgomery informing her the that Castle will be sticking around the station since he will be writing more Nikki Heat books. When both hang up, Beckett accuses Castle of setting the whole thing up and Castle is swearing he had nothing to do with it. While they are arguing, Beckett gets a phone call from. Esposito informs her there has been a murder. As she starts walking away, leaving Castle standing there, she stops, turns around, and asks him if he was coming or not. They continue arguing as they walk away.