A sailor from the USS Foster is found dead in waters off the Bahamas, eerily standing at attention dressed in his dress whites with a sword at his side. The NCIS team finds that the Foster is definitely missing 19-year-old Seaman Russell MacDonald. At first, Kate (SASHA ALEXANDER) speaks with his mother, who insists that – despite the appearance of MacDonald’s death – he would never commit suicide. Coming aboard the USS Foster, Gibbs (MARK HARMON), Tony (MICHAEL WEATHERLY) and Kate begin questioning his shipmates, only to find he had virtually no friends and seemed to be consumed in his duties as a computer technician in the Foster’s combat information center. His Chief Petty Officer Laura Caruso did believe he seemed troubled lately and she reveals he had some mysterious friend with whom he was obsessed. And clearly, by his fellow shipmates’ accounts in the tech center, MacDonald – though talented – was focused elsewhere. Further, searching MacDonald’s rack, Gibbs mysteriously finds a book titled “The Japanese Art of Iaido – The Ultimate Sword fighting Guide.”
Then, as Ducky (DAVID McCALLUM) examines the corpse and Abby (PAULEY PERRETTE) pores through forensics related to the death, they discover that the knife on MacDonald was sharp – though it shouldn’t have been – and had been used. And a chain around his waist was attached to two 25-pound weights that obviously dragged him down. Abby also discovers a piece of paper with some notes for some “character charter” that she believes must have been part of a fantasy computer game in which he was an on-line role player. On Gibbs’ orders, Abby begins delightedly searching for the violent on-line game that consumed MacDonald. Tapping into “The Immortals” – which seems to fit MacDonald’s interests such as swordplay -- Abby infiltrates it at a high enough skill level to retrieve information about the players. MacDonald went by the screen name of Weylin, and he was battling another player -- who they learn was also aboard the Foster – by the screen name of Kinvaras. Further, Abby’s infiltration at that level allows her other links to the players, including a diary for Weylin – if she can crack his password. After she does, she finds a litany of ramblings written by MacDonald, which she immediately passes on to Kate.
Deducing that MacDonald and his rival were taking the game to the next level, they realize that the pair were staging sword fights aboard the Foster. In investigating weapons on board, Tony discovers that the Navy Exchange in Puerto Rico – where the Foster originated -- sold illegal swords to both MacDonald, and his shipmate Ron Zuger.
Questioning Ron, the agents discover that the two were engaged in computer game combat that led to sword fighting, and that he was MacDonald’s rival. Mixing reality with fantasy, MacDonald was proving increasingly dangerous, so Ron – after MacDonald injured him -- opted for the computer game again. Still, MacDonald’s death is looking increasingly like a suicide. At the time MacDonald jumped off-board, Zuger has an alibi, and Ducky says there was no sign of struggle before his death. But Zuger finally comes forward with a sad revelation: while Ron thought they were just playing a game, MacDonald was convinced otherwise. And when MacDonald tried to kill him, Zuger challenged him to prove his immortality: take his sword and swim to shore... wearing weights. The deranged MacDonald – convinced he was immortal – readily agreed.
While the mystery of MacDonald’s death is solved, the diaries have led NCIS to a possibly deadly aftermath. Paranoid, the deranged MacDonald – with his plan to reach shore -- was also apparently intent on destroying his shipmates’ leader. Trying to unscramble his diary entries, Gibbs deduces that he has planted a bomb in Commander Russell’s quarters. He charges into Russell’s cabin and hurriedly removes him, seconds before the bomb detonates.