A woman Marine in an outlying area of family housing aboard the Marine Corps Base, Quantico, Virginia, finds bloody water spraying from a shower head; Gibbs and company investigate; in a holding tank in the water-supply system, Ducky and others find both a body and the URL of a website, which yields clues. Palmer says that something is missing, and Duck says that it disappeared while the victim was still alive. Gibbs and Tony find a second body with a second URL and a personal greeting; a girl singer helps. The clues suggest more deaths. Gibbs receives a strange phone call; Ducky makes a psychologic profile on the unknown bad guy, and McGeek makes a significant discovery. A second man dies. Gibbs calls a suspect to his chat room, where the suspect runs into an unexpected problem and yields to it. The gang figure out the game, then they show up at the next site, where Gibbs realizes who the real bad guy is, so he and the others nab him.