HMS Perseus, a British submarine, vanished in the Mediterranean during World War II with the loss of nearly her entire crew. One man, John Capes, later surfaced with an account so strange that the Royal Navy and Capes himself kept parts of it quiet for decades, including suspicions that he had been working undercover. Underwater investigator Richie Kohler leads a team back to the wreck site to reconstruct what actually happened aboard the sinking boat and how a single sailor could have escaped from a stricken submarine in deep water. The film mixes wreck footage, dive investigation, and reenactment to walk through the explosion, the flooding compartments, and the escape itself, checking Capes' own postwar testimony against what the wreck and the historical record can support. Interviews with historians and divers fill in the submarine's design, the tactical situation in the Mediterranean at the time, and why the loss went largely unexplained for so long. The result is part disaster investigation, part character study of a survivor nobody fully believed.