At an outdoor black-tie party, Jack Rhodes smiles at Gillian Bromley from across the lawn, and follows her inside the adjoining mansion. There, he strikes up a conversation, impersonating Cary Grant, but Gillian accuses him of sounding like Tony Curtis, doing an impression of Grant. When Jack’s friend Sheila introduces him to another party guest, Gillian disappears. He soon follows Gillian to the second floor, then traverses the mansion’s exterior. Perched by a window, he watches her steal expensive jewelry from a hidden drawer. He then positions himself at the bottom of the main staircase, hoping to interest her in a date. Jack is undeterred by Gillian’s lack of interest, and suggests that they will meet again. Later, Jack returns home with Sheila, but leaves her sprawled on the downstairs bed while he investigates a noise in his bedroom. When Jack discovers Gillian in his closet with the intent to rob him, he locks her inside and returns to Sheila. Once Sheila is gone, Jack questions Gillian, who admits to stealing for the thrill. They arrange to meet for lunch, but before she leaves, she returns his stolen billfold. Later at Scotland Yard, Gillian meets Chief Inspector Cyril Willis, who plans to use Gillian to trap Jack, the world’s most brilliant diamond thief. Cyril is determined to outfox Jack before his mandatory retirement starts. Cyril’s dream is to leave the force in a blaze of glory by apprehending Jack. Later, Gillian arrives for lunch with Jack in a stolen white convertible Mercedes sports car. When a police car flags her for speeding, she evades the law. Gillian and Jack dine and gamble at a casino. Still later, Jack cooks Gillian a Chinese meal in the kitchen of a Chinese restaurant that he owns. As they make plans to see each other again, Gillian tells Jack that her former boyfriend, a diamond merchant named Maxwell Levy, is fond of sleeping with her for good luck when he sends a large shipment of rough diamonds to be cut in Antwerp, Belgium. Otherwise, her interest is in Jack. Later, Gillian reports back to Cyril that Jack is apathetic about Maxwell’s business routine. However, Cyril believes that Jack will fall for their trap and reminds Gillian that the reward for her help will be erasing her police record. The next day, Jack takes Gillian to a Wimbledon tennis match between Bjorn Borg and Jimmy Connors. While enjoying a snack courtside, Jack’s friend Ronnie Taylor and his friend, "Maxwell Levy," stop by their table. When Maxwell invites them to watch Tracy Austin play, Jack encourages Gillian to join the men. She later learns that Jack paid an actor named Peter Pritchard to impersonate Maxwell, because her story struck him as false. Caught in her lie about Maxwell, Gillian reveals Cyril’s plan. Jack is angry about the deception, but wants to know why Gillian has not slept with him. After confessing her reticence to arbitrarily sleep with a stranger, she invites Jack into her home, and they make love. Later, Jack tells Gillian that he wants to intercept Maxwell’s diamond shipment. He orders her to telephone Cyril for the shipment date, but the phone line is busy. At a meeting with Cyril, Gillian tells the inspector she no longer wants to assist him, but he describes the miserable conditions of prison and she changes her mind. Cyril tells her he will pass along the date of Maxwell’s shipment in a couple of days. Meanwhile, Jack enlists his friend, Nigel Lawton, to handle logistics of the robbery. Jack also recruits Ernst Mueller, a former German WWII pilot, and Ferguson, a former pilot turned entertainer. Soon, Cyril calls on Gillian to get more details about Jack’s whereabouts, but Gillian claims ignorance. When Jack arrives at Gillian’s house, he is introduced to the inspector. The men have a cordial conversation, and then, the inspector leaves. Later, Jack and Gillian join Mueller and Ferguson aboard a private jet. Jack and Gillian, disguised as Arabs, arrive in Antwerp. They change clothes, but hand their disguises to decoys, who are followed by several local detectives. Meanwhile, Cyril speaks with Antwerp Inspector Vanderveld, who reports that his men have Jack and Gillian under surveillance. Later, Jack shows Gillian a getaway airport security car hidden in a barn. When she complains that she has not been apprised of Jack’s master plan, he says he never tells his associates everything, but declares that he loves her. Elsewhere, Cyril watches his team load the diamond shipment in a van, which is then transferred to a private plane bound for Antwerp. However, Mueller and Ferguson, flying a second plane, pretend to be air traffic control operators. They radio the pilots of the first plane, instructing them to divert their aircraft to Amsterdam, Holland. When Cyril meets the plane in Antwerp, he thinks the diamond shipment is being unloaded, but the disguised second plane lands with fake jewels. The real shipment goes to Amsterdam, where Jack and Gillian, disguised as security guards, meet the plane and steal the diamonds. Soon, airport security chases after Jack and Gillian. Soon, Gillian eludes the men and drives the getaway car to a private airstrip, where the second plane piloted by Mueller and Ferguson waits for them, and they escape. Elsewhere, Maxwell Levy inspects the loose diamonds and declares them to be worthless quartz stones. On a yacht sailing in tropical waters, Jack and Gillian enjoy cocktails, but Jack reveals that the diamonds they stole in Amsterdam are fakes. When Gillian asks who has the real diamonds, Cyril climbs aboard and proposes to sell the actual diamonds to Jack for $1 million. Jack offers $3 million to Cyril for his troubles. Cyril and Jack explain that they collaborated from the beginning, and Gillian deduces that the diamonds were already stolen before she and Jack took possession of the shipment in Amsterdam. When Gillian remarks that the men have pulled off a robbery worth $30 million, Jack replies that the only problem is that $30 million will not buy what it used to buy.