Castle's agent, Paula Haas (guest star Debi Mazar; who's also been Dancing With The Stars in Season 9), tells him he's being tapped to re-launch a major franchise, that of a certain British secret agent. She doesn't mention any names, but were thinking the main character likes martinis. At the book launch party for HEAT WAVE, Beckett discovers that Castle called her “extraordinary” in his dedication. They both have mixed feelings about Castle's new offer, as their working relationship looks like it may be coming to an end. Think they'll be able to squeeze in one more case?
The body of Eliska Sokol is found down a manhole. At the victim's apartment, Castle and Beckett find a picture of a woman with a young boy at a playground. The mother and son in the picture are Melissa and Zane Talbot. Eliska had worked in their building until she was fired as a result of an incident with Mrs. Talbot. Melissa and her husband, Dr. Cameron Talbot, were upset that Eliska had given Zane a lollipop without her permission, as they didn't really know her all that well.
When they track down Eliska's ex-husband, Teodor Hajek, he bolts, fearing deportation. After a highlight reel-worthy takedown tackle by Esposito, Hajek is arrested. He had no idea his ex-wife was dead and emotionally tells of how his relationship with Eliska went south after the death of their young son.
Hajek's alibis out and the team finds a witness who was out of town during their first canvass. The witness saw her neighbor arguing with a man the day Eliska was killed. A police sketch artist helps her create a composite of the man. We've seen this guy before. His name: Cameron Talbot. Yes, Dr. Talbot was having an affair with Eliska. He went to see her to break things off and give her hush money. Beckett isn't really buying it, but Dr. Talbot's nurse confirms that he was in the office at the time of Eliska's death.
The team finds out that someone paid Eliska's rent after she died and a blonde woman has been coming by to pick up her mail. Did we mention Mrs. Talbot was blonde? Anybody up for a stakeout? When a blonde woman arrives, we see that it isn't Mrs. Talbot. It's the nurse from Dr. Talbot's office. He asked her pick up the mail so that he wouldn't be embarrassed when the police discovered a check he'd mailed to Eliska.
But Esposito and Ryan discover the Talbots and Eliska had crossed paths a long time ago. Eliska and Mrs. Talbot both gave birth to sons and at the same hospital on the same day. It's the same hospital that employed Dr. Talbot. Eliska gave the lollipop to Zane so that she could swab the boy's mouth for a DNA test. The results of that test were in the Eliska's mail. They revealed that Eliska was the true mother of Zane, not Mrs. Talbot.
Dr. Talbot admits that their child had a rare congenital disorder and he switched babies. When Eliska showed up and threatened to reveal the truth, Dr. Talbot killed her. After Dr. Talbot's arrest, Beckett and Castle bring Teodor Hajek to the Talbot's apartment. There, they witness the joy of Teodor Hajek being introduced to his son Zane by Mrs. Talbot. It's a pretty touching moment when Teodor helps his son repair his toy train.
Castle and Beckett try to find a way to say goodbye now that they they've solved their final case together. But then Castle gets a call from his agent. HEAT WAVE is a hit and his publisher wants a three-book deal. Beckett's phone rings, too. It's Montgomery telling her the mayor would like her to continue working with Castle. Beckett thinks Castle may have orchestrated this entire turn of events but there's no time to bicker as her phone rings once more. There's been a murder. And just like that, Castle and Beckett are back on the case. Don't you just love a happy ending?