Ryan and Espo investigate the death of a man who died locked in the trunk of a car that was set on fire.
Beckett, Castle and Vikram intercept the call from LokSat and go to a drop sight to try and make an ID with Hayley in tow. Espo calls Beckett to tell her that the body in the trunk was Caleb Brown. Beckett realizes that the drop sight is a trap set by LokSat.
Mason Wood, the head of the GDS drives up in a food truck and saves them. He says Rita, Castle’s step-mother sent him to protect them. Beckett heads back to the precinct where Vikram has told Ryan and Espo about LokSat. Caleb left a clue in the trunk of the car writing “RTHDMN” which they believe means Right Hand Man.
They get video of what they think is Caleb meeting his right hand man and then follow him to a house in the suburbs but it turns out to be empty.
Castle is in lockdown at the P.I. office with Hayley, Alexis, and Martha but worries about Beckett and decides to go to the 12th to be with her. He is abducted along the way by the same man who killed Caleb Brown. Mr. Flynn uses chemical truth agents to find out about everyone who knows about LokSat and then kill Castle. Then LokSat arrives. It’s Mason Wood. The truth serum forces Castle to tell him that Beckett, Hayley, Vikram, Espo, Ryan, Martha and Alexis all know about LokSat. He has tears streaming down his face knowing they will all be killed.
Espo and Ryan track the phone connected to LokSat and arrive in time to save Castle but Mason is already on the way to kill Beckett. Castle uses the truth serum on Mr. Flynn to find out that Mason is bringing Beckett to the CIA building to kill her but he can’t contact her because they’ve cut off all communication.
Castle finds a way to break out and finds Beckett and Mason in the basement. Beckett is able to disarm Mason and she and Castle are reunited.
After the case is wrapped up, Castle and Beckett go home but it turns out that Caleb Brown isn’t really dead. He breaks in and shoots them both.
Then we flash forward to seven years later where Castle and Beckett are happily living in their loft with their three children. Always.