As NCIS mourns the loss of Ducky, the agents find comfort in working on one of his unfinished cases involving a woman whose father was dishonorably discharged from the Marines.

Gibbs and McGee head to Alaska while the team works at home to uncover the conspiracy behind the serial killer.

NCIS, FBI and MI6 continue an international manhunt for an escaped British spy who is targeting current and former agents.

During an NCIS hit-and-run investigation, Palmer joins a stranger on the outside ledge of a building in an attempt to save his life.

Season 15 begins with the NCIS team tirelessly searching for Gibbs and McGee two months after they were last seen fighting an elusive group of rebels in a Paraguay jungle. Meanwhile, Vance and Torres are summoned to a congressional hearing regarding the fallout from the Paraguay mission.

As the hunt for Ziva's replacement begins, Gibbs and his remaining team members must head back to work with the past still weighing heavily on their minds.

After a malnourished and confused 9-year-old is found hiding in a storage unit, NCIS reopens an old missing persons case when the girl is believed to be the daughter of a Navy recruit who disappeared 10 years ago. Also, Bishop discovers notes that Special Agent Ziva David kept on the case long after NCIS officially stopped investigating.

Ernie Yost, an elder Marine veteran confesses to murdering his best friends almost 60 years ago in a war battle and the NCIS's job is to solve all the secrets and save Ernie from prison.

When Tony opens a mysterious letter thinking it's for him, a small puff of white powder comes out and it releases a possible deadly virus. Kate calls for help and soon she and Tony are isolated in a bio-hazard isolation room for precausion. McGee and Gibbs work to discover who sent the envelope and how to cure their friends before its too late for them. Meanwhile, Tony's health gets worse...

A group of serial terrorists seek revenge and send Ari (guest star Rudolf Martin) on an assignment to kill Gibbs. The NCIS team goes on a hunt after him, but when they discover that Ari has a bigger plan, it is already too late... the NCIS team will have to face the death of one of their team members...

When a murder investigation leads the team to Gibbs' hometown, secrets about his past are revealed.

Terrorism shakes the foundations of the Navy and NCIS to such an extent that Gibbs and the team face surprises that will devastate them.

After Ducky is attacked by a man claiming to have information about his half-brother who died decades ago, the NCIS team searches for the perpetrator, as Ducky recalls the final emotional days he had with his sibling. Also, with the holidays approaching, Bishop and Jake discuss the future of their relationship.

NCIS investigates when the body of a murdered Navy lieutenant, who disappeared over a year ago, is found by a cemetery grounds crew while relocating caskets on the property. Meanwhile, Gibbs and McGee must pass a psych evaluation before they can officially resume working.

After Torres and Bishop are victims of a hit-and-run, Torres fights for his life in the Intensive Care Unit.

NCIS comes under attack by a mysterious enemy from the past. Also, Vance tries to mend fences with his estranged son by explaining why, despite the ongoing dangers of his job, he still chooses to stay at NCIS.

Still stressed over Kate's death, Gibbs is even more hurt when Ari kidnaps Ducky and decides to play with his mind. Gibbs sends Tony to follow Ziva, and after Jenny and Gibbs are almost shot to death, he is even more determined to finish once and for all with Ari, and Ziva's strange connection to Ari will be just the help that he needs.

While the NCIS team investigates the link to a dead journalist and a Navy petty officer, Ziva is taken aback by the sudden emotional visit by her father, who she hasn't seen in more than two years. While Ziva questions the purpose of her father's visit as the Director of Mossad, her father insists he is in the U.S. strictly to see his daughter for Shabbat dinner.

The NCIS team investigates whether a fire on a U.S. Navy ship that served as a secret detention site for indicted terrorists was accidental or an intentional diversion for an escape. Meanwhile, Gibbs travels to his childhood home following news of his father’s passing.

The NCIS team re-examines an ambush on a group of Special Ops snipers in Iraq after an American couple is attacked in the same area six months later. When Gibbs discovers he needs the account of the lone survivor, Marine Gunnery Sergeant Aaron Davis, he tries to connect with the wounded warrior who is being treated for PTSD at Walter Reed Medical Center.

After the terrorist's attack, the NCIS team is shocked and is trying to cope with Kate's sudden death, each in their own way. Now, their mission is to collect all the possible info on Ari and find a way to stop him before he does any more damage. Meanwhile, while feeling guilty over Kate's death, Gibbs is informed that the NCIS director is leaving, and the replacement is Jenny Shepard, a woman Gibbs worked with in the past and had a romantic history with.

The NCIS team must work on an assignment against a terrorist group without Gibbs, who is still recovering from the explosion. Gibbs has to fight with memory loss, while he's visited by his mentor and his former boss. His help is needed, expecially when the team discovers that Gibbs is the only person who has informations about the terrorists' plans, so he needs to retrieve his memory before the terrorists attack.

When a Navy Lieutenant becomes the victim of radiation poisoning, the NCIS team must find the person who's behind the crime. The Lieutenant, who is working for the IAEA (International Atomic Energy Agency), claims that only his two colleagues knew about his assignment, and whoever it was, the person tried to stop the Lieutenant's next investigation.

Ziva's homecoming to Israel with Gibbs and the team is not a happy reunion with her father, the enigmatic and powerful head of Mossad. As tensions rise, Gibbs is forced to make a decision that sends shock waves through NCIS.

In the aftermath of an event that strikes close to home, the NCIS team unites to seek answers and demand justice in order to protect their most vulnerable colleagues.