Phillip Calvert, a British Treasury secret agent, is sent to investigate the hijacking of five cargo ships in the Irish Sea, tracing the latest vessel—the Nantesville—laden with £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).
Posing as marine biologists, Calvert and his partner Hunslett find the locals suspicious and hostile. They begin to suspect Cypriot tycoon and shipping magnate Sir Anthony Skouras, whose luxury yacht Shangri-La sits offshore, might be behind the piracy of the gold bullion. While sweeping the surroundings aboard a Royal Navy helicopter, Calvert befriends a group of remote shark fishermen who seem friendlier than Torbay’s skewed locals. He also encounters the occupants of a nearby castle, where Lord Kirkside and his teenage daughter behave oddly and act with hostility toward the newcomers.
As the helicopter ferries Calvert back to Torbay, it comes under sudden shore-fire and the Royal Navy pilot is killed. The craft crashes onto the rocky shoreline, explodes, and slides into the sea. Calvert escapes the sinking helicopter and returns to his research yacht, Firecrest, only to discover that [Hunslett] is missing and that his boss, Sir Arthur Arnford-Jones, known as “Uncle Arthur,” has unexpectedly appeared. Together they brace for action, fighting off boarders and preparing to sail again. On raising anchor they find Hunslett’s body bound to the dragging line.
They are soon joined by Charlotte, Skouras’s wife, whom they pull from the sea after she claims to have escaped his yacht following abuse. Calvert is drawn to her, yet remains wary of her story and motives. A pirate speedboat approaches, and Calvert rams it, shoots the occupants, and blows the craft up in retaliation for Hunslett’s death.
To neutralize Skouras and his modern-day pirates, Calvert enlists the aid of the shark fishermen. He suspects that the missing bullion ships are being sunk to smooth the way for an invisible offloading of gold. Diving into the bay, he locates the Nantesville and fights a diver he suspects killed Hunslett, killing him in the fray.
Back on land, Calvert secretly enters Kirkside’s castle and questions the Lord’s daughter, only to learn that Skouras is an innocent victim and that his real wife is being held hostage with other locals in the castle’s dungeons. He then slips into the underground dock where the gold is being offloaded.
At midnight, eight bells, the shark fishermen slam their boat into the underground dock gates, catching the pirates by surprise. The pirates had anticipated the attack because Charlotte has been covertly broadcasting Calvert’s plan to them via radio. She is revealed to be the wife of the ringleader, not Skouras. A fierce firefight erupts, the pirates are wiped out, and Calvert allows Charlotte to escape—with one bar of gold in her possession.
