After staying at an English B&B, American tourists George and Martha Wilson unaccountably find themselves followed by a group of inept espionage agents. The agents, having mistakenly given a roll of film containing stolen plans to George and Martha instead of the intended couriers, are now trying every trick they can think of to get it back, but without much success. Eventually George and Martha realize that out of all their film, one roll has already been developed. George is intrigued by what he sees on the film and becomes obsessed with finding out what it all means. It's obviously a complicated piece of machinery, but what it its purpose? The agents, the real couriers, George and Martha, the police, and even MI-5 get involved in the chase, as they travel from one tourist attraction to another, finally ending up at the family reunion at Uncle Albert's farm. In the surprise conclusion, everyone at the gathering, even the farm animals, get involved in apprehending the thieves and solving the mystery.