Complications arise when the NCIS team discovers a link between a murder victim and the current boyfriend of Gibbs and Agent Fornell’s ex-wife. Meanwhile, NSA Analyst Ellie Bishop adjusts to joining the NCIS team.

The NCIS team suspects Gibbs' arch-nemesis is behind an explosion at a global terrorism summit. Gibbs elects to sit out the investigation in order to help a grief-stricken Fornell, whose life is on the verge of imploding following his wife’s murder.

The NCIS team, alongside the FBI and MI6, continue an international manhunt for an escaped British spy who has left one colleague fighting for their life in ICU.

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

After the NCIS team tracks evidence from the murder of a Navy captain to Ducky’s deceased mother, Ducky looks back and questions a pivotal life choice he made 37 years ago. Also, the team exchanges holiday gifts, and Gibbs spends Christmas dinner with Fornell.

Former FBI Agent Tobias Fornell pleads with Gibbs to do whatever is necessary to take down drug dealers and suppliers in the area after his daughter, Emily Fornell, is hospitalized from an opiate overdose. Gibbs is haunted by the personal aspects of the case and his history with vigilante justice.

Gibbs and Fornell's crusade to find the head of the drug ring responsible for Fornell's daughter Emily's overdose comes to a climax. Also, Gibbs and Vance finally let the team in on a case that has dangerous implications.