While in prison, Herbie Hassler, a small-time American gangster, devises a scheme to rob an English mansion called Great Friars. After his release Herbie and his friend Marty Miller persuade Nick Marco, a powerful gang leader, to back their plan. They immediately fly to England to complete the preparations. Herbie, posing as a tourist, takes a guided tour of the house and meets the owner, Lady Sophie Fitzmore, and her nephew Freddie. Herbie and Freddie become friends, and when Herbie claims to be an expert in security systems, Freddie invites him to stay on at the house. Freddie explains that the family has become financially unstable and is concerned about the safety of their heirlooms. Herbie helps plan tourist attractions and completely forgets about the robbery, until Nick arrives and orders them to proceed. Unable to persuade Nick that Great Friars has tight security, Herbie himself successfully defends the mansion against the gangster. Meanwhile, Lady Fitzmore has gone to a London gambling casino and recouped the family fortune. She returns to Great Friars with the intention of transforming the mansion into her own casino.