While a group of friends are out sailing, one of them goes swimming and discovers a body at the bottom of the ocean. Seaman Russell McDonald's body was in found in the Bahamas, which is near Puerto Rico. That gets Dinozzo started on how much he loves Puerto Rico. McDonalds body had a sword and had chains with weights hooked to them wrapped around his waist. Dinozzo and Todd go to tell McDonald's mother about him. His mother told them about his father and how he dies when he was 12. She also told them that he wasn't the type of person who would kill himself. Gibbs, Todd and Dinozzo settle into the ship that McDonald was in and start investigating. Everyone tells them that McDonald kept to himself. Agent Gibbs interviews Petty Officer Carnahan whom tells him he didn't know him. Agent Dinozzo interviews McDonalds C. O. who said that McDonald was very intelligent but hasn't been focused lately. So far, he is the only one who believes McDonald would kill himself. Agent Todd talks with his Medical corpsman who stated that McDonald was troubled but doesn't believe he would kill himself. She also tells her about how he was obsessed with a person who was there, but wasn't really there. So, he had a sword he wasn't suppose to have, he was found at the bottom of the ocean with weights, his C.O. believes he that would commit suicide but his mother and his corpsman doesn't. He was also teaching himself how to use that sword from a book Agent Gibbs found in his rack. On a video line, Abby informs Gibbs, Todd and Dinozzo that she found a St. Christopher medal and a character charter for a fantasy game; a massive multi-player on-line role-playing game MMORPG. She also noticed that the sword had nicks in it, possibly was in contact with another sword. So Todd goes investigates that while Abby will try to find out what type of game McDonald was playing and what type of files he had in that game. Gibbs and Dinozzo talked with Ducky next. He found dirt under McDonald's fingernails; the dirt was from the sea floor. With that, he comes to the conclusion that McDonald was still alive when he went into the water. Later on, Dinozzo finds out that McDonald probably got the sword from the Base Exchange Roosevelt Roads Naval Station and the only way he could get one if the cashier wasn't checking IDs. No one is allowed to have swords on board, so if he had one, he would have to hide it. Meanwhile, Gibbs talks with a captain and is informed that if two of his men wanted to stage swords fights and not get caught, they would probably do it in, Damage control, in the machine room at night. While Abby works on MMORPG, Todd talks with the corpsman again to find out if there were any injuries reported that could help. Later, Dinozzo informs Gibbs about the swords and Abby calls Gibbs and tells him that she found a game site called "The Immortals" McDonald frequently went to. His character name was Waylin. The charter she found on him referred to his main rival knight Kinvaras. Their main mode of combat was "Parle", which means Sword Fighting. She also discovered that the guy who plays Kinvaras, logs on from the foster, the ship that Gibbs, Dinozzo, and Todd are on. Gibbs believes that McDonald found out who Kinvaras was and that he was on the ship and decided to take it to the next level. So, if they both had swords, chances are they had to have bought it at the Naval Base Exchange in Puerto Rico. So Dinozzo offers to go down to Puerto Rico, pretend to buy a sword, expose the clerk and find out what he can. Back at Navy NCIS headquarters, Abby finds McDonald's "Weylin's" Profile on Immortals and now has to find out the password to access the information. Meanwhile in Puerto Rico, Agent Dinozzo cons a clerk into selling him a sword and then tells her he's really Navy NCIS and needs to find out who else she has sold swords to. Down in the control room, Gibbs and Todd find scratches on poles; scratches that look like a sword hit it. While discovering more of them, Todd gets into a conversation about McDonald being a Catholic and that he would never commit suicide. Being that she's Catholic herself, she would know because it's suicide is not part of their culture. Dinozzo calls Gibbs and informs him that the Base Exchange was selling swords illegally. They did sell one to McDonald and to three others who Dinozzo has a description of. Later, Gibbs and Todd go through each Seaman's file that matches the description Dinozzo gave them. They run across Petty Officer Ronald Zuger file and learn that he had a cut on his arm from a glass but there weren't any glass shards in the wound. He also matches one of the descriptions that Dinozzo provided. Gibbs finds him on the computer playing a sword-fighting round in Immortals. Meanwhile Abby gets access to McDonald's password on Immortals and skims through his numerous journal entries. She then calls Gibbs but Todd picks up instead. Abby informs Todd that she got into McDonald's journal; Todd offers to read all of the entries. During the questioning, Gibbs learns that McDonald and Zuger did fight for real and it was McDonald's idea to take the game to the next level. He also finds out that Zuger was helping a friend with a computer problem the night McDonald went overboard. Meanwhile Todd reads McDonald's journal entries and learns just how obsessed he was with the game. Gibbs questions Carnahan and asks him about the computer problem that Zuger helped him with the night McDonald went overboard. He learns that he was helping him delete porn off of his computer because he was sure his C.O. was suspecting him of doing just that. On a video communication line, Gibbs talks with Ducky about any other injuries McDonald may have, Ducky tells them he nothing else new. So with that, it does look like a suicide but Todd disagrees, and not because of religious morals. After reading McDonald's journal entries, she learns that McDonald was so intent to finish his battle with Kinvaras that he wouldn't let anything stand in his way. So with that, he wouldn't have a reason to kill himself. While Dinozzo requests Zuger's records, Todd discovers an entry that says when McDonald is off the ship; he will release a great plague on a realm by the next full moon destroying all of his enemies. With that, Gibbs remembers that McDonald applied for NBC program (Nuclear Biological and Chemical Weapons) but failed. So they figure the realm McDonald was referring to was the ship; McDonald was convinced that the crew was aiding his enemy. The group also realizes that the sun is going to set in less than an hour. So Gibbs gets Dinozzo to get Zuger to find out if there was anything on McDonald's computer about any kind of nuclear bomb. Gibbs knows that Navy Ships don't have the authority to carry bio-chemical weapons nor have the materials to make one and with that goes to warn the captain. While Seamen search for around the ship for the bomb, Dinozzo gets Zuger on the web-site to find out if McDonald said anything about setting off a bomb on the ship and Todd and Gibbs learn that there are no missing parts from the ship to create a bomb. While on the computer, Zuger's Kinvaras files get deleted due to a virus McDonald set. Gibbs questions Zuger if Mcdonald said anything about setting off a bomb on the ship; Zuger tells him he doesn't know anything. Gibbs gets Todd to look over McDonald's diary again to see if they missed something and has a chat with Zuger to find out everything that happened between him and Mcdonald. As more time passes by, the bomb still hasn't been found, nothing else has been found in McDonald's diary and no other ship or aircraft will arrive in time to help unload everybody off the ship. Gibbs finally gets a confession out of Zuger. Turns out that Zuger realized that McDonald really thought he was immortal and was trying to kill him when they fought for real. So Zuger challenged McDonald to swim across the ocean wearing the weights because that was the only way McDonald could prove to him that he was really immortal and because he wanted to win the game. Gibbs gets called into the Captain's Sea Cabin and Todd still searches for a clue in McDonald's journal entries. With a little over five minutes, Todd puts the pieces together and comes to the conclusion that when McDonald was referring to destroying the realm and cutting of the heads of the crew, it meant two different things. Destroying the realm meant destroying Kinvaras's web-site and cutting of the heads of the crew meant killing the leader of the crew and the leader of the crew is the captain. McDonald was so convinced everyone on the ship was aiding his enemy, the only way to get revenge was to kill the leader. As the sun is about to set, Gibbs and the Captain are arguing about the bomb and Todd and Dinozzo comes bursting in to find out where the captain usually is during sunset; which is in the room they are in now. So the four get out of the room in time before the bomb goes off. Back at Navy NCIS, Abby explains the situation to Ducky. Seaman Mcdonald is Weylin and Petty Officer Zuger was Kinvaras. Weylin and Kinvaras battling online in the Immortals game. Zuger always wins and McDonald challenges him to fight for real; when they do, McDonald wins. So Zuger goes back to the one thing he knows he can beat McDonald in, Immortals. Then, Zuger realizes that McDonald really thought he was Waylin and challenges him to prove it. So McDonald puts on the weights, jumps into the ocean and tries to walk across the ocean floor to dry land. In the end, Dinozzo gives Todd and Gibbs their gifts from Puerto Rico. Todd gets a bikini (really a bottom and a hat) and Gibbs gets an online fighting game in Spanish. With that, another case is solved.