A serial killer named The Shark is terrorizing London by killing his victims with a speargun and then, dressed in a scruba-diver's wetsuit, using the city's sewer tunnels to make his getaway. Inspector Wade of Scotland Yard is assigned to the case, and he begins to suspect that the Shark is tied in with a smuggling ring the authorities believe is run out of a dive on the Thames waterfront called The Mekka.