American insurance investigator Scott Walters is sent to London to interview businessman Perry Henderson about his US$2 million insurance policy leaving his elderly mother as sole beneficiary. Walters meets and is taken with Perry's personal secretary Marian Taylor but wishes to speak to Perry. His brother Arthur Henderson explains that Perry is deep sea diving off the coast of Portuguese East Africa but doesn't tell Walters he is after deposits of diamonds on the sea bed. Alarmed by the danger, Walters tells Arthur to make Perry stop all dangerous activities or he will forfeit his policy.