Two youths, David and Andrew, enjoy their holidays at Old Rusty's farm in a wild and lonely valley. But after a new windmill power generator is built, something changes. Strange sights and buzzing sounds begin to occur; Old Rusty's ham radio is subject to crackling sounds. Then, while outside at night, they are swooped by a large flying 'shadow', which leaves them all fearful throughout the night. The next morning they discover a weird additional 'sail' on the windmill - in the shape of a giant insect's wing.