London gangster George Thomason and his right-hand man, Ken Pile, an animal lover with a stutter, plan a jewel heist. They bring in two Americans: con artist Wanda Gershwitz and weapons expert Otto West, a mean-spirited anglophobe. Wanda and Otto are lovers, but they hide this from George and Ken, pretending to be siblings, so Wanda can work her charms on them. The heist is successful, and the gang escapes with a large sum in diamonds. They hide them in a safe in an old workshop. Soon after, Wanda and Otto betray George to the police and he is arrested. They return to collect the diamonds, with Wanda planning to double-cross Otto, as well, but find that George has moved them. In Ken's fish tank, Wanda discovers the key to the safe deposit box containing the diamonds and hides it in her pendant.
Wanda decides to seduce George's barrister, Archie Leach, so he can persuade George to plead guilty and give up the location of the diamonds. Archie is in a loveless marriage and quickly falls for Wanda; Otto is jealous, and his interference causes Wanda and Archie's liaisons to go disastrously wrong. Wanda accidentally leaves her pendant at Archie's house, which Archie's wife, Wendy, mistakes for a gift for her. At Wanda's insistence, Archie recovers the pendant by staging a burglary. Eventually, Archie, feeling guilty, ends the affair.