A Russian diplomat’s son, Grigory Mishkin, has his throat slashed in a warehouse. The Russians ask for a Russian diplomatic security officer to be assigned to the case with the detectives at the 12th precinct. Vasili Sernoff is a huge fan of Castle and Nikki Heat. He is heartbroken that Castle and Beckett no longer investigate together.
Grigory was involved with some diplo-brats who frequently ignore the law because they have diplomatic immunity. Jurgen, dropped him off at the building where he was killed and gave him the the owner’s manual for a Jeep Cherokee. Turns out that Grigory’s mother, Anya was killed in a car accident in a Cherokee.
The 12th’s investigation uncovers that Anya was meeting a man at a bed and breakfast. The security camera footage helps them identify the man as Frank Thomas who is really former Russian spy Anatoli Arcadi. Anya was also a sleeper agent here in the U.S.
Beckett asks Castle to keep Vasili distracted so he takes him out sight seeing but it turns out that Vasili is really Russia’s best cleaner agent. He’s figured out that Grigory stumbled across a possible attack on U.S. soil by Russians and it may have gotten him killed. He plans to torture Anatoli but Castle figures out that they were trying to sneak someone into a Russian Diplomatic party through the catering staff before Vasili has to torture anyone.
Vasili gets Castle, Beckett, Espo and Ryan into the party. Kate sees a red laser through a window and tries to stop a sniper attack but Sergei, Grigori’s fire gets hit anyway but will survive.
Kate calls in Rita, secret agent and Castle’s step-mother. Rita confirms that everyone thinks that Castle and Beckett have separated. Then Rita and Vasili reconnect and help one another on the case.
Beckett finally tells Castle that Rita is his step-mother and that she didn’t tell him because he was trying to protect him. Castle tells her that that story is getting old. When he finally meets Rita he admits to flares of anger over all of the secrets but he loves Beckett. He asks if he’ll ever see Rita or his father again but Rita says no. Then she hugs Rick and tells him that his father is very proud of him.
Through security footage Espo finds out that Anya, Grigory’s mother fakes her death and is alive. They pick her up and Beckett interrogates her. Anya was the sniper. She was trying to kill Sergei who wasn’t Grigory’s biological father. Sergei was selling Russian secrets to the highest bidder and when Grigory caught on, he killed him.
Due to his diplomatic immunity, Sergei thinks he’s going back to Russia to retire but Vasili tells Castle and Beckett that he’s actually being sent to live out the rest of his days in the coldest place in Russia as punishment for his sins.
Castle gives Ryan a present for the baby shower but only Beckett gets to attend the shower due to their “separation”