Biff Brewster and his British schoolmate, lively Peter Balsam, have been eagerly planning a bicycle trip to Peter's grandmother's home in Devonshire, with a stopover at the Atomic Laboratory where both their fathers are involved in an ore extraction problem. The boys' itinerary has been carefully mapped out so as to include the sights of London as well as the beautiful English countryside.
But strange happenings keep threatening their trip: a mysterious postman connected with a stolen mailbox--a trenchcoated shadow on their trail--the puzzling purchase of a rare book--a queer little scientist--the ominous black-bearded man...
As Biff and his pal cycle along, they become increasingly aware that something they don't understand is going on, and they seem to be in the center of it. Inspector Duggin of Scotland Yard thinks so too, but it is not until the boys wind up their exciting journey with a desperate adventure on the moor that the truth finally comes to light.