It is a fantasy tale of His Majesty's Submarine Scorpion vanishing during the Second World War, leaving only one crew member (David Jason) surviving.

A Royal Navy crew along with the sole survivor of the original voyage is given the mission of retracing the last days of the boat prior to her 1943 disappearance.

HMS Scorpion, a British submarine that had gone missing in the Baltic Sea during the Second World War, surfaces in the path of a Soviet freighter in 1981.

He and Alan Cassidy, one of the vessel's designers, join a Royal Navy crew on a mission to retrace Scorpion's last days before she went missing.

The mission will take the submarine into Soviet waters while a surface ship, HMS Oakland, escorts the boat.

Hardy realises that the "Soviet submarine" was destroyed at the exact same place and time where the Scorpion sank a German U-boat in 1943, and speculates that "the past is breaking through".

The aircraft are Second World War Luftwaffe fighters; Captain Byrnes says in stunned disbelief "This can't be real".

Cassidy and Hardy find no bullet damage on the deck or tower from the strafing attack, and conclude that Travis may have shot the captain to preserve the mission.

Modern Soviet officers detect the incoming hostile vessel, and begin preparations for a nuclear war.

In the original story Candlefish fell through a Geo-magnetic Anomaly (GMA), one of ten, five in the Northern Hemisphere, five in the Southern; and surfaced in the middle of another.

The final mission of the Candlefish was changed by her captain, to an unplanned entry to Tokyo Bay, in an attempt to sink anything Japanese, and thus acquire a higher figure for tonnage sunk.

This falls rapidly apart as the Candlefish sinks, with one survivor – Jack Hardy – injured and washed overboard from the conning tower.