Vyšetřovací tým pracuje na záhadném utonutí poddůstojníka na křižníku. Muž byl nalezen v moři se závažími kolem nohou a šavlí u boku. Překvapením pro vyšetřovatelé je zjištění, že mladíka natolik pohltila počítačová online hra, že začal věřit ve svou nesmrtelnost. Na důkaz toho, že je skutečně nesmrtelný, skočil se závažími a šavlí do moře. Dojít na pobřeží se mu však skutečně nepodařilo.

We find ourselves in the squad room, looking over Gibbs' shoulder at the dead sailor on a computer screen. The dead man is identified as Seaman Russell MacDonald, age 19. The ship MacDonald was assigned to had just left Puerto Rico heading to Norfolk. Tony talks for several minutes about how much he loves Puerto Rico. In Autopsy, we learn that there is no reason for MacDonald to be in dress whites, and as an enlisted man he should not have an officer's sword. Further, the "ceremonial" sword has been sharpened to a razor edge. Ducky is given a few minutes to wax poetic about the sword and of driving in England. Gibbs sends Kate and Tony to talk to MacDonald's mother.

Tony is having trouble with the idea of having to talk to the bereaved mother. From this interview we learn that MacDonald's father was Scottish, a member of the Black Watch, and his mother believes he was very happy and had many friends. He may have "had his quirks," but Mrs. MacDonald is emphatic that Russell would not have killed himself. This may have something to do with the MacDonald family's staunch Catholicism (suicide is a mortal sin for Catholics).

The team takes a helicopter out to the USS Foster, which was MacDonald's duty ship before he went swimming with weights. The party line seems to be that MacDonald kept to himself; no-one wants to talk about him. Kate is dismayed to learn that the three of them are expected to share sleeping quarters. Gibbs volunteers to take the couch and leaves to investigate MacDonald's bunk. DiNozzo interviews MacDonald's CO, who says that MacDonald worked in the computer center on the ship and despite being good at his job, had recently become unfocused. The Lieutenant believes MacDonald may have killed himself. Kate interviews the ship's doctor, who confirms that MacDonald was having a hard time adjusting and that she suggested he go into counseling. She also states that MacDonald had a mysterious friend with whom he was obsessed. She does not think MacDonald would commit suicide. In MacDonald's bunk, Gibbs finds a book about Japanese sword fighting.

Via videoconferencing, Abby reveals that a St. Christopher medal and a role-playing game character generation sheet were found on MacDonald's body. She believes the game in question is an MMORPG (Massively multiplayer online role playing game). Kate guesses that evidence, if any exists, will be on the Internet rather than on MacDonald's physical computer, since Abby believes "if he was any good, he won't leave any footprints." Abby has also found the sword blade had been nicked from contact with hard metal, such as another sword. Ducky confirms MacDonald drowned.

Tony interviews an ensign about the sword found with MacDonald. He confirms that all of the officers' swords are accounted for. They discuss where MacDonald might have got the sword; the obvious answer is the base exchange at Roosevelt Roads, in Puerto Rico. Gibbs interviews the ship's commander; the commander says that a good place for sword fights would be damage control, in the machine shop, after hours.

Back in Abby's lab, she is playing games at work. She's supposed to be - Abby believes she has found the game MacDonald was playing. She's a little too knowledgeable about it to be a casual gamer. Ducky stops by, tries to think of a story to go with her game, and comes up blank. On the Foster, the medical corpsman is going through her records for Kate and looking for suspicious lacerations that may have come from clandestine sword fighting. Ducky brings Abby a Caf-POW! and we learn that the game is called "The Immortals." Abby explains that if she can penetrate the innermost stronghold of the dungeon, she can get a log of all characters who have played this game. She is brutally killed by an Orc during this explication and must begin again.

On the ship, Gibbs gets a call from Abby. She has succeeded in obtaining server logs and confirmed that MacDonald played The Immortals as a character called Weylin, whose main rival was another character called Kinvaris, and that their primary mode of combat was sword fighting. She has also learned that Kinvaris' player was located on the USS Foster with MacDonald.

Tony suggests going to Puerto Rico, buying a sword undercover, busting the clerk, and then "swapping" the bust for information on who else bought a sword. He wants 24 hours. Gibbs gives him six. In her lab, Abby finds "Weylin's" personal online journal. It is password protected. In Puerto Rico, Tony makes his undercover buy and busts the clerk - or tries, as she begins throwing clothing at him and screaming at him in Spanish. (Probably not anything Tony's not used to.) In the machine shop, Gibbs and Kate investigate some marks on metal poles which appear to have been made by contact with a sword.

They discuss the case. Kate is adamant that MacDonald would not have committed suicide based on the mother and the medical corpsman's testimony. Gibbs understands that Kate's own Catholic upbringing will not allow her to consider that MacDonald may have killed himself. She insists it's her cultural insight into Catholic families and not personal beliefs coloring her opinion. Tony calls from a bar in Puerto Rico to say that MacDonald definitely bought his sword at Roosevelt Roads, as well as three other seamen whose description Tony has.

Kate and Gibbs sit, going through files and matching descriptions to see if anyone matching the description of a sword buyer shows up on the list of suspicious lacerations that the medical corpsman gave Kate. They find a suspect - Petty Officer Ronald Zuger, who works in the computer center and supposedly cut his arm on a plate glass window, but there were no glass shards in the wound. Gibbs and Kate bust Zuger playing The Immortals in the computer center. Gibbs takes him away to interrogate him. He confirms that Zugler is "Kinvaris."

Abby cracks the password into "Weylin's" diary and calls Kate. She explains that MacDonald was pretty much a whack job. Kate volunteers to take over going through the diary. In interrogation, Gibbs begins to query Zugler about the sword fighting. Zuger denies it until Tony comes in with Zuger's unauthorized sword. Zuger seems to equate himself with Kinvaris in that creepy I-think-I'm-actually-my-character sort of way that gamers always seem to get in Hollywood (and in real life, I might add). He also has an alibi - helping a friend with a computer problem. Kate goes through MacDonald's diary and finds out that Abby was right - MacDonald was a whack job. He expresses paranoia that the whole crew is on "Kinvaris'" side and conspiring against him. The diary also mentions a "final triumph" and "the element of surprise."

Gibbs checks Zuger's alibi: it is MacDonald's bunkmate, P. O. Carnahan, who has been downloading porn onto his work computer and needed Zuger to clean it off before his CO busted him. Ducky, via video conference, confirms that MacDonald does not seem to have been in a struggle before he went overboard. Gibbs and Tony are fairly sure it was suicide; Kate is insistent that there's more to it. Gibbs thinks she means his Catholicism; she says she means the diary. MacDonald was intent on continuing to try and overcome Kinvaris, whom he has never beaten. Kate wonders "why someone so intent on his mission, however deranged, would off himself and end it." She continues reading in the diary that MacDonald planned to escape the ship and release a great plague against the realm as the sun sets beneath the next full moon. We are more sure than ever that MacDonald was a whack job.

MacDonald had applied for the Nuclear, Biological and Chemical Weapons program. He failed the psych evaluation. Gibbs becomes sure that MacDonald planned to blow a dirty bomb on the ship. Turns out, this is the date of the full moon and they have about an hour before the sun sets. Gibbs alerts the ship's commander and the red alert sirens begin to sound. Tony grabs Zuger and sits him in front of a computer to check his site for any clues MacDonald may have left. Kate checks with Homeland Security about any possible missing materials to build a dirty bomb; Homeland Security says they don't think so, but they'll check. (That's comforting.) In the computer center, Zuger checks his site only to find that MacDonald has hacked into it and made it go kerflooey. He breaks down, near tears about the loss of Kinvaris. Gibbs gets in his face and jerks him back to dealing with the problems going on in reality, since Kinvaris doesn't really have to worry about anthrax or smallpox.

Kate and Tony start going through the diary again, looking for more clues. The ship is being searched for bombs, and evacuation helicopters are on their way from other ships. Gibbs gets a little physical with Zuger and the truth comes out: MacDonald had gone off the deep end and decided that he really was Weylin and that Zuger really was Kinvaris. Zuger didn't kill him per say; he told MacDonald that if he really was immortal, he should walk across the ocean floor to dry land. When Gibbs wants to know why Zuger would let MacDonald do that, Zuger puts on his crazyface and says, "To win the game."

As the sun sets, the commander demands Gibbs report to his sea cabin. Kate and Tony discuss what MacDonald said in his diary about cutting off the head so the body will die. It occurs to Kate that the "plague" could simply be a computer virus. They deduce that because of MacDonald's paranoia, which extended to the crew, the threat of "cutting off the head" may actually mean killing the ship's commander. They bolt out of the computer center.

Cut to: The Commander's sea cabin. He is reading Gibbs the riot act about the ship being in an uproar. Because that's so important when we might all die of the plague. Kate and Tony burst in, demanding to know where the commander is at this time every day. He says he is here, in his sea cabin. Kate says "We gotta go. NOW!" and they run, Gibbs forcing the commander to run as well. The sun sets, the group get a few feet down the hallway, and the commander's sea cabin is blown, if not sky high, definitely into a state of sudden renovation.

In Abby's lab, Abby is trying yet again to explain the crazy gamer situation to Ducky, who is having as much trouble as the rest of us understanding why someone would think they were a character on the computer. We take a few minutes to allow Abby to recap the entire episode for us. Cut to: the ship, where our intrepid Agents are packing their stuff to go home. Kate is having trouble (as we all are) with the blurring of the line between reality and fantasy for MacDonald. Tony states he's glad his parents pushed him into sports at high school. For about the third or fourth time, Tony offers the souvenirs he brought back from Puerto Rico. Gibbs, tired of hearing about it, allows Tony to play Santa.

In Kate's bag is a "Puerto Rican bikini": bottoms and a matching hat. Gibbs asks if there's any chance she's going to try it on; Kate says "You first" and throws it at him. Gibbs, "Trust me, it's not gonna fit." Kate: "Pigs, I work with pigs! In Gibbs' bag is a MMORPG starter kit... in Spanish. Kate has a great smile. They leave; Tony returns briefly to retrieve the bikini bottoms.