Based on the TV series the film follows DI Regan and DS Carter as they became embroiled in a deadly political scandal. One of the leading members of the government, Charles Baker (Ian Bannen) is about to secure a huge deal with OPEC stabilising the world oil market and boosting Britain's position within it. Baker is the rising star of government, regarded a future prime minister, who is closely controlled by his urbane, manipulative American press secretary Elliot McQueen (Barry Foster). When Regan investigates the mysterious death of a prostitute as a favour to one of his informants, he becomes aware that Baker and McQueen might be involved. A spate of killings follow - which sees Regan take on both the criminals and the hierarchy of the Metropolitan Police and British security services.\
The plot is set on a group of bank robbers, who are both violent and successful, strangely getting away each time with an amount around the £60,000 mark, and often leaving behind cash in excess of this sum. The robbers are willing to kill their own team, to get away. As Jack Regan himself puts it after the first raid in the film: \"I've never seen so many dead people\". Armed with gold-plated Purdey shotguns, they evaded Regan and the Flying Squad for quite some time, before Regan finds encouragement from his Detective Chief Superintendent who was sent down for corruption because Jack wouldn't testify in court for him.\
For years after her father's disappearance, the beautiful Miss Mary Morstan receives anonymously every year one large and valuable pearl. When, finally, an invitation arrives to meet the mysterious pearl-giver, Mary Morstan seeks Holmes's advice, and he begins to unravel some remarkable clues, including a poison dart and a man with a wooden leg. What Holmes finds is an old soldier who is determined to punish those who betrayed him and his friends by stealing their oriental treasure.