Gibbs sends Bishop and Quinn to Philadelphia to work with MI-6 Officer Clayton Reeves on the murder case of a British operative. Meanwhile, Quinn deals with the NCIS case that caused her to leave the field.
The gang mourn the death of Kate; they recall her and deal with her death. Ari puts a round into Abby's lab, and the NCIS gets a new director, who shares a history with Gibbs. Ducky accepts an invitation to an unusual meeting. [Continued.]
Ducky meets Ari, Tony follows Ziva, Ari secretly betrays Israel and Mossad, Ziva passes cash and a fake passport to Ari, Gibbs and Jen seek Ari but miss him, Ari finds Gibbs, and Ziva finds them. The gang attend Kate's funeral in Indiana.
Gibbs again questions a condemned serial killer whom he arrested 10 years before. The killer, three days from execution, has offered to reveal the locations of the bodies if he gets to talk with Gibbs. The gang solve that and much more.
Ziva returns. A 140-year-old iron casket shows up at the Smithsonian Institution; it contains the body of a Marine staff sergeant on a UA, so Gibbs and company investigate. The team chase the clues, follow a map, and figure out the story.
A sailor dies of a gunshot and the resulting car wreck. Gibbs and company investigate; the victim first appears to be an impostor, but Abby shows otherwise. The impostor is another man. The gang figure out the death but not everything.
Two wives of two deployed enlisted Marines disappear; the NCIS investigates. McGee recalls something, and that gives the team a start. The two wives went into business together online, but it goes out of control. The gang figure it out.
A six-year-old boy sees who kidnaps his father, a Naval officer who works with a highly classified online-encryption system; the boy calls the NCIS, so Gibbs and the team investigate and eventually solve the crimes and recover the father.
Tony and Ziva go undercover and under the covers as assassins, a married couple, who recently died, but who apparently have a contract to execute at the Marine Corps birthday ball in Washington. The NCIS gang figure it out and work it out.
The amputated legs of a young woman turn up aboard the Marine Corps Base, Quantico, Virginia; Gibbs and company investigate. Forensic evidence implicates Tony because a pro has framed him. Abby works hard, and the gang find the bad guy.
Three supermodels take part in a reality show aboard the Marine Corps Base at Quantico; one of them disappears but shows up dead on a fence; the NCIS investigates. The victim's boyfriend shows up dead at a motel. The gang figure out all.
While investigating at a port, Tony and Ziva encounter unexpected criminal activity; they become trapped inside a shipping container. Gibbs and McGee, with the help of Ducky and Abby, find not only their teammates but also the bad guys.
A woman Naval officer becomes abducted, and she calls NCIS on her cellphone; Gibbs and the gang investigate. She's involved with both a movement of nuclear-fuel rods and an anti-pervert group. The team find her, and they solve the case.
Two Korean wives of Marine sergeants die in execution-style shootings; while the NCIS investigates, a third such wife disappears. The team find the wife, then identify the shooter and figure out the shooter and the changes in that person.
While busting three Navy enlisted men running an automotive chop shop, Gibbs, Tony, and Ziva find a severed head in a cooler in the trunk of one of the cars. The gang find the rest of the body, the story, the answers, and the bad actors.
An ambulance ignites and explodes after picking up the body said to be that of a Marine lance corporal who died due to a car wreck; Gibbs and company investigate, partly to determine the identity of the body. Gibbs and Jen close the case.
In the Shenandoah National Park a bear eats a part of the body of a murdered Navy petty officer; Gibbs and company investigate. A woman companion has disappeared; the gang deduce her identity. Ziva finds her, and Gibbs nails the killer.
A student, Kody, seizes control of a room and six classmates, threatening to detonate a bomb; the NCIS and others respond. Gibbs walks into the room and confronts Kody, who demands to see his mother, said to be dead. The team work it out.
A boy at play on a frozen pond sees the body of a man below the surface. Gibbs and company investigate; they find three more bodies. The first was a Marine; the others were gang members. The team figure it out; Gibbs provides a solution.
Tony and Ziva go to the home of a woman Navy lieutenant and cryptologist; they find her dead, apparently by suicide. Gibbs and the gang find evidence of murder instead; after pursuing the wrong clues, they lose a mole but catch the killer.
A couple enter a room at a Navy Lodge and find bloody human body parts; the Gibbs team investigate. Gibbs and Ducky agree that the scene was a setup. An imbalanced former boyfriend stalks Abby. The gang chase the clues and figure them out.
A suspect collapses and dies while in the custody of Ziva; she becomes a suspect rather than an investigator. The team investigate. Then the director disappears, and her driver dies. Ducky clears Ziva, and Gibbs and his gang rescue Jen.
Aboard a Turkish-flag ship an explosion occurs, killing one person and leaving Gibbs comatose with burns, abrasions, and a concussion; the team investigate. Eventually Ducky welcomes Gibbs back, but Gibbs does not remember him. [Continued.]
Gibbs regains consciousness, but he's lost his memory since 1991. He starts to remember a few things; Jen helps, then an old buddy, then Ziva. Gibbs gets back his memory and a solution, gives a severe warning, becomes ignored, then quits.