 A badly decomposed uniformed male body is found in a barrel of hydrofluoric acid on a naval base. His skull was fatally struck with a blunt instrument. A partially preserved tattoo indicates a submarine crewman, yet none is reported missing, so NCIS suspects the murderer took over a sailor s identity after killing him. Leroy Jethro Gibbs and Special Agent Caitlin "Kate" Todd, most (un)welcome as unprecedented female on board, conduct an investigation on board the sub Philadelphia, on NATO exercise in the Atlantic, to find out about five possible suspects among the fifteen new crew members, including sonar Petty Officer Drew and Petty Officer Thompson, who lied about missing a year of high-school because of juvenile detention. The captain, Commander Robert Peters, is unwilling to keep them isolated as he can t spare time or anyone during the strenuous exercise, but sabotage is a real danger, probably within 48 hours as that time frame suffices for DNA identification. The roughly reconstructed face doesn t resemble any of the five, nor do their fingerprints turn up anything- the service personnel files must be doctored; Ducky thinks of looking for crew group photographs from ships they served on- still no match, but Abby Sciuto thinks up one better. Meanwhile on land, Anthony DiNozzo and his apt pupil Timothy McGee examine someone who suspiciously left the base recently, who turns out to be a whales-obsessed eco-terrorist with a terrifying use for chemicals on board...