Under the Polar Ice-Cap... The atomic submarine 'Dolphin' has impossible orders: to sail beneath the ice-floes of the Arctic Ocean to locate and rescue the men of weather-station Zebra, gutted by fire and drifting with the ice-pack somewhere north of the Arctic Circle.

But the orders do not say what the 'Dolphin' will find if she succeeds – that the fire at Ice Station Zebra was sabotage, and that one of the survivors is a killer…