When a plane lands in London, England, a passenger is loaded into an ambulance, but as the vehicle drives away, the man and his accomplice congratulate themselves for stealing a briefcase filled with diamonds. As they lift the jewels from the case, however, the ambulance explodes. Sometime later, Meecham, the director of Consolidated Selling System, a wholesale diamond clearinghouse located at 11 Harrowhouse, learns from his assistant, Coglin, that the theft has been dealt with. Meanwhile, American diamond salesman Chesser and his girlfriend, Maren, race through the English countryside in Maren’s red Ferrari on the way to 11 Harrowhouse. Back at the System’s vault, Meecham instructs his appraiser, Watts, to price Chesser’s diamonds at a twenty percent mark up of their true value and complains that Chesser does not live up to his father’s model of a good buyer. When Chesser meets with Meecham and Watts, he realizes they are trying to up sell him but agrees to the deal. As Meecham leaves the room, Watts apologizes. That night, Chesser puts the diamonds in a peanut butter sandwich and Maren protests that he does not stand up to Meecham. One evening, the couple is followed by a stranger and Chesser is presented with a letter from Massey, a wealthy industrialist who seeks his assistance. While escorted by limousine to Massey’s country manor, Chesser explains the man is both a war hero and a criminal. At lunch, Massey introduces the couple to his companion, Lady Bolding, and tests Chesser’s ability to distinguish between real and counterfeit diamonds. As the ladies leave the table, Massey challenges Chesser to find a $1 million, 100-karat diamond in one week. Returning to 11 Harrowhouse for the stone, Chesser is observed by Meecham and Coglin through a one-way glass and Coglin suspects that Maren, a wealthy widow, is the buyer. With Watts’s assistance, Chesser selects a large, uncut diamond and invites Watts to come to his apartment. When they meet, Watts speaks about himself in the guise of a “companion,” explaining that he has terminal cancer and will die just short of completing thirty years at the System, the amount of time required for obtaining retirement benefits. Although the System knows about his predicament, they will only give him twenty percent of his earnings and he will not be able to leave money to his family. Upon Watts’s recommendation, Chesser takes the diamond to Wilderstein, a Belgian gem cutter, and offers him $100,000 to finish cutting the stone by his week deadline. After picking up the diamond, Chesser and Maren are shot with tranquillizer darts in Maren’s Ferrari by two men disguised as road workers. When one of the men searches Maren’s bra, Chesser mutters that the stone is under the hood. Back at Massey’s estate, Chesser reports the theft and speculates that it was orchestrated by Wilderstein’s men. He vows to retrieve the stone and refuses to pay back the money, but Maren writes Massey a check for $1 million. As she leaves for the pool, Chesser rips up the check. Enticed by the view of Maren swimming naked with Lady Bolding, Massey invites Chesser to stay at his home. Later, Chesser watches film clips shot by Massey’s investigators and identifies Max Toland as the diamond thief. Unsurprised, Massey reports that Toland is an informant for the Consolidated Selling System. Massey claims that the System, which has acquired 80 million karats of diamonds but released only 60 million to control demand and inflate prices, has $12 billion of stones stored in a vault beneath 11 Harrowhouse. Later, as Maren and Chesser float on a raft in the pool, Maren suggests that they outwit the System and steal the $12 billion stash of diamonds. That evening, Massey says that he has an office next door to 11 Harrowhouse. He proposes that Chesser use the location to orchestrate a heist of the System and agrees to pay $15 million. As Maren and Chesser drive away, Massey opens a box on his desk to admire the stolen diamond. Back in London, Chesser gives Watts a Consolidated Selling System check for £250,000 in return for information about the underground vault. Chesser reads Watts’s report out loud to Maren as they lie in bed and learns that the roof of 11 Harrowhouse has several areas that are not connected to the security system. That evening, Maren and Chesser explore the rooftop for access and Maren sneaks along the drainage gutter to find the building’s electrical unit. Back at Maren’s house, Chesser paints cockroaches different colors, then returns to the roof of 11 Harrowhouse and unleashes the insects through the electrical ducts to determine which one leads to the vault. The next morning, Watts uses a piece of chocolate cake to attract the roaches and, after finding a red bug, reports back to Chesser. The following evening, Watts sneaks back into the vault after signing out. As he opens the drawers of diamonds, he swallows pills with alcohol. With Watts’s assistance, Chesser threads a vacuum hose through the electrical duct and Watts suctions the diamonds back through the hose to a delivery truck, where Maren collects the stones in a padded box. A security guard reports the truck to his colleague, but the man attributes it to the renovators working at night in Massey’s office. At the completion of the job, Chesser and Maren drive the diamonds to a construction site near an airfield while Watts shaves and drinks in the vault. That morning, Meecham arrives to discover Watts at his desk, inebriated. Watts announces that he ate the diamonds and shows Meecham the empty drawers. Outraged, Meecham explains that the System maintains diamond scarcity for good reason and the prices of the stones will plummet worldwide unless the stolen gems are recovered. Uncooperative, Watts falls to the floor, dead, and Coglin announces the death a suicide by strychnine. Meanwhile, Chesser and Maren load a sample of the diamonds into a bag and drive to Massey’s estate to collect their earnings. They are met by Toland and Massey’s armed henchmen, but as they race away in the van, the gates close and they are unable to escape. Despite Lady Bolding’s protests, Massey and his men go after the couple on horseback. As Chesser and Maren are shot at by an army of pursuers, Lady Bolding gives chase in her sports car and offers to lead them off of the grounds. Maren admits that the woman is in love with her. After a long chase, Massey waits for the van with his rifle, but the vehicle careens over a wall and the couple escapes. Lady Bolding gives them her car, but they continue to be followed. As a last resort, Maren drives through a cement drainage pipe in a construction zone, and their pursuers, who are unable to make it through, explode in their vehicle. Massey admits defeat. Returning to their stash of diamonds, the couple contemplates their next move. When Maren suggests they keep the bag of diamonds worth $20 million, Chesser argues that if they do, they will be no better than the System. The couple mixes the box of diamonds with cement and pours it into a fountain that decorates the courtyard of 11 Harrowhouse and Massey’s office. Sometime later, the couple has a modest wedding.