While replacing a furnace at Quantico, workers find a smoked body in the chimney; Gibbs and company investigate. Fornell shows interest, saying that the victim was a serial killer. Abby enables Gibbs and Fornell to confront the real killer.

While testing a robotic driverless SUV, a woman Navy lieutenant becomes trapped inside it, and she dies of carbon-monoxide poisoning; Gibbs and company investigate. McGee suggests murder, Abby has trouble, and the gang find the bad guy.

An illicit arms dealer reveals a deal involving another dealer; McGee and Ziva tail a key player, who dies. Ducky impersonates the dead man and meets the Frog, whom Jen has long pursued; much good results, but the gang do not get the Frog.

Jen meets with an informant about the Frog, but someone kills the witness; Gibbs and company investigate. Jen obsesses about catching the Frog. Abby produces many answers; although the gang persist, they find a surprise but no Frog.

While Jen attends a conference in Paris, Gibbs serves as the acting director. When a taxicab delivers a fare to the NCIS headquarters, the passenger, a Yemeni, is DoA; the gang investigate. Jen diverts to Moscow, and many answers emerge.

Jen returns from Paris; at home she finds a curious sight. All NCIS agents in Washington must take polygraph tests; Gibbs and the gang look into that. Tony and Jeanne deal with an injured pedestrian; Jeanne introduces Tony to her father.

Tony meets Jeanne's father; Tony excuses himself to move his Mustang, which explodes while the gang watch; Ducky says that the victim is not Tony. The Frog meets Jen, but she lets him leave; Gibbs and others pursue him but arrive too late.

Fornell and his Hoovers arrive in the squad bay; he tells that the Gibbs team is under investigation for the murder of the Frog. However, the Gibbs gang investigates the FBI, and they put the pieces together and explain them to the suits.

On a sidewalk a former fiancée fatally stabs a petty officer; while Gibbs and company investigate, a different woman accuses Ducky of a war crime and stabs him through one hand. Ducky surrenders, but Gibbs sorts it out and gets Ducky free.

Kort asks Gibbs and the team for help in hunting one of NCIS Most Wanted, but it's more complicated when his accountant enters the picture.

EJ and her team investigate yet another P2P murder; the victim is an NCIS agent from Norfolk; Gibbs and company pitch in, as does Franks. Kort, a CIA agent, spills in a briefing. Both Franks and EJ encounter the prime suspect.

Gibbs and Tony follow EJ and her team to a nearly vacant building; they find that the P2P murderer has killed Levin, seriously injured Cade, and kidnapped EJ; Gibbs pursues the suspect and EJ; he finds her, left to suffocate. McGeek traces a phone number to CI-Ray (Ziva's boyfriend). Ducky, with help from Gibbs, explains the changes in the MO of the killer. From Levin's left forearm EJ secretly removes a small object. Ziva tries to intercept Ray, but Cobb, the suspect, intercepts and kidnaps her; later, while Gibbs, Tony, McGee, and EJ are present inside a hotel room, Cobb arrives, enters, and surrenders, but he refuses to tell where he put Ziva. However, Abby provides clues to a place, where the gang plus Ray find and rescue Ziva, who gives explanations. Back at the HQ, Cobb escapes with two hostages; Gibbs and others follow; a confrontation ends the career of the P2P killer. The SecNav resigns, and his successor gives Tony a highly sensitive and classified task.

The team continues the international manhunt for an escaped British spy while a colleague fights for their life in the ICU.

The NCIS, FBI and MI6 teams continue the international manhunt for an escaped British spy who's targeting current and former agents. Anthony DiNozzo bids farewell to NCIS.