When heavy fog wreaks havoc among air travellers throughout southern England, outspoken Cynthia Beeston (Margaret Rutherford) - a forceful proponent of "Positive Thought" - insists on being taken from London Airport to Blackbushe Airport, where she might be able to fly to Dublin. Harassed airline employees find emergency relief coach 13 and reserve driver Percy Lamb (Frankie Howerd) - so hapless he cannot find his way around the airport, much less the roads - to transport her. She is joined by mild-mannered Henry Waterman (Toke Townley), pulp-thriller addict Janie Grey (Belinda Lee) and Ernest Schroeder (George Coulouris). To satisfy a regulation, stewardess "Nikki" Nicholls (Petula Clark) is assigned to shepherd them. Rounding out the party is airline first officer Peter Jones (Terence Alexander), who hitches a ride. Unbeknownst to most of them, robbers have stolen £200,000 worth of gold bullion from the airport bonded store and hidden the proceeds in the boot of the coach.
Two of the crooks are caught; under questioning by Inspector Henley (John Horsley), one breaks down and admits the gold was stowed on the coach and that the mysterious and notorious "Banker" is the mastermind. Henley informs Percy by radio, but the fog is so thick, Percy has no idea where he is. In mid-call, Peter pokes what Percy thinks is a gun into Percy's back and tells him to keep driving. They wind up at a deserted booby-trapped village used by the Army for training.
When Schroeder finds a Sten gun, Peter grabs it. Schroeder then informs him that it does not work, and produces a pistol of his own. After a scuffle, it turns out that Peter is working for airport security, while Schroeder is a policeman. Miss Beeston - the Banker - ends up with the gun, and her henchman Henry tries to start the coach. Percy saves the day, having removed the rotor from the engine, and knocking the pistol out of Miss Beeston's hand with a stone.
Val Guest had first met Frankie Howerd backstage in his dressing room at the London Palladium where Howerd was topping the bill in a long running variety show. Guest says that Howerd was reluctant at first to make a film - "Films? Oho, that's a dangerous game. Get your name up there outside a cinema and if they don't go in it's all your fault! Thank you but no thank you!" Howerd finally agreed, but made three demands. Firstly, Guest had to write a comedy-thriller, so that if the comedy part didn't work, the thriller part might. Second, he did not want his name first above the title. And finally, he wanted his favourite comedy actress Margaret Rutherford to be in it. Shooting took five weeks, with a budget of £45,000.