A decomposing body is discovered in a tub of acid at a naval base. Ducky and Abby discover that he was a Submariner by the tatoo on his arm. Gibbs discovers that there were no reports of missing crew members so there has to be an imposter. Gibbs and Kate go onto the submarine that they believe has the imposter on. They interview the five new submariners to try to find which one is the imposter. Tony and McGee find out the real name of the imposter and go to his house and find that he is going to release a biological weapon in the submarine to stop them from harming the whales. They tell Gibbs that and so the airconditioning system is shut down. Abby finds a photo of all of the new submariners and finds out it is Drew who is the imposter. Knowing that NCIS know he is an imposter, the fake Drew suffocates himself but not before he swallows the capsule with the biological weapon in. They put his body into the freezer and the capsule opens letting the gas out. Gibbs realises what they have done and they shoot him out through a torpedo tube before the gas gets out of Drew's body.