Modesty Blaise, a secret agent whose hair color, hair style, and mod clothing change at a snap of her fingers is being used by the British government as a decoy in an effort to thwart a diamond heist. She is being set up by the feds but is wise to the plot and calls in sidekick Willie Garvin and a few other friends to outsmart them. Meanwhile, at his island hideaway, Gabriel, the diamond thief has his own plans for Blaise and Garvin

The script, which was written on a day-to-day basis as the film was being shot, concerns the adventures of a motley crew of swindlers and ne’er-do-wells trying to lay claim to land rich in uranium deposits in Kenya as they wait in a small Italian port to travel aboard an ill-fated tramp steamer en route to Mombasa.