Minus Five picks up uranium readings which appear and disappear. A trawler hauls in a catch of fish which could be radioactive, but a more important 'catch' is contested with spear guns under the ship as the Endeavour crew stumble upon uranium smugglers.

The Endeavour is assigned by the government to chart unknown channels in the Barrier Reef. A man once badly mauled by a white pointer shark seeks revenge in the shark's breeding ground and endangers the lives of the Minus Five divers.