The body of a doctor is found in a park in D.C. and the NCIS Major Case Response Team are summoned to the scene to investigate. It appears to be a normal investigation that is until Director Vance arrives and informs that the team that the killer they're looking for is someone he's been hunting down for the last twenty years....
A nurse who had threatened the life of the doctor who is the initial victim (the one whose killing tips Vance off to Kai's presence). She shoots at McGee and DiNozzo with a shotgun, thinking the doctor sent them, is arrested and interrogated but cleared.
The press release for the episode "Endgame" confirmed that this storyline will indeed be a crossover episode with the character of Lee Wuan Kai being introduced in the NCIS: Los Angeles episode entitled "Killshot" on 20 October 2009. Endgame concluded the crossover, and aired on 10 November 2009.
Lee Wuan Kai arrives in Washington D.C, having been treated for injuries she received while fighting Vance and the OSP team in the NCIS: Los Angeles episode, "Killshot".
Vance is seen in the field for the first time.
Vance reveals he has been hunting Kai for twenty years.
Special Agent Chad Dunham returns to give the team an insight into Kai's past.
Vance's full name is revealed to be Leon James Vance.
It's revealed that Abby is allergic to avocados.
McGee meets a girl named Amanda.
It is later revealed that Amanda, McGee's new love interest is actually a South African assassin named Juliet Tippon who was hired by the North Koreans to kill Kai. She threatens McGee who she believes knows where Kai is but in a twist, Kai shoots Amanda who soon dies in McGee's arms.
Kai reveals that she wants Vance to kill her to end the pain she is feeling at having been brainwashed and trained into becoming an assassin since early childhood.
Vance attempts to help Kai but Jackie shoots Kai dead.