The film follows Jack Hopkins (played by Michael Craig), an aircraft designer with a passion for traction engines. His boss (played by Cecil Parker) is eager to sell a new supersonic jet plane that Jack has designed to American millionaire Paul Fisher (Alan Hale, Jr.). The first encounter between Fisher and Jack goes badly, and tensions only heighten after Fisher's daughter Kathy (Anne Helm) damages Jack's prize traction engine "The Iron Maiden", rendering it impossible to drive solo. Jack is desperate to enter the annual Woburn Abbey steam rally with the machine, but his fireman is injured and unable to participate. When all seems lost the millionaire himself is won over by Jack's plight and joins him in driving the engine; the two soon become firm friends..