After the assassination of one of their agents in Amsterdam, Sir Gerald Tarrant, Harry Andrews recruits Modesty Blaise, Monica Vitti, a former criminal mastermind, to safeguard a shipment of diamonds en route to Abu Tahir, the Sheikh of a small Middle Eastern kingdom. The shipment has also drawn Gabriel, Dirk Bogarde, the head of a criminal organization that includes his accountant McWhirter Clive Revill and bodyguard Mrs Fothergill Rossella Falk. Modesty believes Gabriel, who maintains a clandestine compound in the Mediterranean, is dead, but he reveals himself to her.
In exchange for an exclusive discount on the kingdom's oil exports, the British government delivers periodic diamond shipments to the Sheikh. Modesty, who has an ongoing love–hate relationship with law enforcement, is recruited not only for her competence, but because she is the Sheikh's adopted daughter and thus trusted by him implicitly. Modesty agrees to the arrangement, on the condition that she is given total immunity by the British government and complete freedom to deliver the diamonds how she sees fit.
With Tarrant monitoring her from afar, Modesty travels to Amsterdam, where she reunites with her former lover Paul Hagen, Michael Craig, a Secret Service agent and aide to Tarrant. She calls upon her longtime partner, Willie Garvin, Terence Stamp, who is reuniting with an old flame, Nicole, Tina Aumont, who may have information on Gabriel through her employer, an illusionist associated with him. Modesty narrowly survives several attempts on her life by Gabriel's assassins, whose failure leads to their swift execution by the ruthless Mrs Fothergill.
Modesty continually toys with Paul, first seducing him before stealing his gun and disappearing.
When Gabriel learns that Nicole is working with Modesty and Willie, he orders her assassinated. The illusionist sends thugs to have her killed, and they succeed when Modesty and Willie fail to intervene in time. Modesty and Willie set themselves up as live bait to draw Gabriel out, but find themselves pursued by Tarrant and a jilted Paul, being briefly arrested before quickly escaping with the help of some smoke bombs. When Modesty attempts to identify and infiltrate the boat being used by Gabriel for the planned diamond theft, she is lured into a trap and captured. Gabriel reveals his true plan, to use Modesty as leverage to force Willie to steal the diamonds for him.
Willie reluctantly agrees to the arrangement, successfully stealing the diamonds from under Tarrant and Paul's noses. He and Modesty are subsequently taken to Gabriel's island fortress, where they are promptly thrown into prison cells. Gabriel offers Modesty to join forces, but she refuses. Willie and Modesty escape and kill Mrs Fothergill, and signal their location to the Sheikh's forces. The Sheikh leads his army to the island, leading to an all-out battle with Gabriel's forces and ending in his capture and the diamonds reaching their intended owner.
In his desert camp, the Sheikh leaves Gabriel tied up outside to dehydrate. McWhirter suddenly appears in Highland dress to free his employer, though no one seems to notice or care. When the Sheikh asks Modesty what he can do for her, she asks for the diamonds. He responds by laughing boisterously and she seems to go along with it, but suddenly breaks the fourth wall by looking directly at the camera.
