An English man by the name of Samuel Colpepper is driving his car to NCIS when he gets assassinated by a man who fires a shell from an AT4 anti-tank weapon.
Colpepper worked for Angus Clarke, a childhood friend of Ducky's. At the time of his murder, he was carrying some confidential Navy information, including dossiers containing information on three sailors, two of whom were dead.
The NCIS team determines that they were all murdered, but their deaths were made to look like either natural causes or accidents.
When Abby realizes that one of the sailors on the list was still alive, McGee and Gibbs rush to hunt him down. As they are interviewing him, he suffers a severe allergic reaction to some shellfish residue which had been put into his water. Fortunately, they get him to a hospital in time.
Meanwhile, Bishop and Ducky head to England to try and find Ducky's now-missing friend Angus Clarke. They want to know why his employee was killed, and why he was killed while traveling to NCIS.
Ducky and Bishop are met in the their hotel lobby by Angus' personal assistant, Gareth Godfrey. As Angus is still missing, Bishop asks if they can search his office in order to find clues which may help find him. Godfrey forbids it, until Angus' wife shows up, and provides permission.
While searching his office, a young woman in a negligee bursts into the room from an adjoining room, and rushes out, while Bishop follows.
Angus Clarke is found at the bottom of a river.
McGee determines that the dead sailors had been in positions of purchase authority for Navy weapons, and that the personnel who replaced them all had the same emergency contact: an Albanian assassin by the name of Gareth Ahmeti, who had been going under the name of Gareth Godfrey - Angus' personal assistant. Godfrey's aim was to smuggle Navy weaponry.
When they find this out, Gibbs calls Ducky to warn him, but it's too late. Ducky and Margaret had been confronted by Godfrey. When Gibbs called, Godfrey took the phone and smashed it against the wall.
Ducky grabbed a scalpel and nicked the brachial artery on Godfrey's arm, causing him to bleed out slowly. Before providing assistance, Ducky demands a confession, which Godfrey provides. This includes the name of the mastermind, Peter Velo
Gibbs and Tony confront Velo, who tries to escape - but he runs directly into McGee's pointed gun and is arrested.
This episode also included numerous flashbacks to Ducky's early days, in which we learn that he had lost his love - Margaret - to his best friend Angus. We also learned that he hates bow ties, but wears them because Margaret happened to like them.