British Treasury secret agent Phillip Calvert is sent to investigate the hijacking of five cargo ships in the Irish Sea, tracking the latest hijacked ship – the Nantesville, carrying £8 million in gold bullion – to the Scottish Highlands and the sleepy port town of "Torbay" on the "Isle of Torbay" (actually filmed in Tobermory, on the Isle of Mull).

They suspect that Cypriot tycoon and shipping magnate Sir Anthony Skouras, whose luxury yacht Shangri-La is anchored off the coast, may be behind the pirating of the gold bullion.

While searching the surrounding area in a Royal Navy Helicopter, Calvert makes contact with a group of remote shark fishermen who appear more friendly than Torbay's locals.

When a pirate speedboat approaches, Calvert rams it, shoots the occupants and blows up the boat in vengeance for Hunslett's death.

Guessing that the missing bullion ships are being sunk to allow the gold to be offloaded invisibly, Calvert dives in the bay and finds the Nantesville.

A fire fight ensues in which the pirates are wiped out, after which Calvert lets Charlotte escape with a single bar of gold in her possession.

An uncredited Charles Gray dubbed the voice of Jack Hawkins, whose larynx had been removed in March 1966 after being diagnosed with throat cancer.

[3] In July 1966 Kastner and his producing partner Jerry Gershwin had purchased five screenplays from MacLean: Where Eagles Dare, When Eight Bells Toll, and three other unnamed ones.

[9] Kastner says he raised the budget by ringing "a guy I read about in Fortune magazine and I went and saw him and said 'I need one point eight million dollars to make this second Alistair Maclean project.'

[1] Kastner wanted a Celtic actor to play the hero Calvert, having had a big success with Richard Burton in Where Eagles Dare.

Bond film stunt arranger Bob Simmons helped him slim down to become a convincing Royal Naval officer trained as a commando and frogman.

The ship still exists, now called the Camara C.[18] The "Northern Diver" the boat used to search for Calvert after the helicopter crash is currently being assessed for restoration by Cam Marine Services on the Isle of Skye.

[1] The Monthly Film Bulletin wrote: "The characters are standard MacLean inventions – tough, unconventional agent and likeable but prickly boss – but the tested clichés look quite fleshy in a setting and climate that manage to be both appropriately inhospitable and recognisably human.

Calvert's dour professionalism and emotional tensions are well set off by the bleak vistas of the Western Highlands and the uncommunicative minor characters ...

The fact that the Panavision cameras roving across glittering seascapes and precipitous cliffs are also visiting the author's ancestral home may be a factor; and Périer does well to keep these landscapes cunningly in view, maintaining a subdued sense of menace across the glassy grey expanses of sea.