 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.