Mrs Sharp, the owner of Castle Farm is angry because some of her sheep have recently gone missing, and she blames the residents of the local youth hostel, run by Mr West. Mr West himself is more suspicious of the neighbouring farmers, the nasty Beasley brothers. Mike Burns the foreman of the farm resigns under strange circumstances, refusing to give a reason. Later Joe, a shepherd boy and Jill, Mrs Sharp's daughter see Mike meeting with the Beasley brothers.

The next day while cleaning out Mike's old room at the farm Joe and Jill find part of a burnt message that mentions blackmail. Worried about Mike Joe goes over to the Beasleys' farm to see Mike under the pretence of returning Mike's forgotten cap and discovers a flock of sheep whose painted markings show that they belong to another farm.

Joe is almost spotted by the Beasleys, but Mike helps him get away. When Joe fetches Mr West to have a look at the sheep in the Beasley's yard, the markings have been changed to show that they belong to the Beasleys. Ted Beasley says he's going to make Joe very sorry for snooping.

A few weeks later the local sheep shearer comes to Castle Farm and secretly hands Joe a coded note from Mike. Joe decodes the note to discover a warning about sheep rustlers.

Joe and Jill hurry back to Castle Farm, passing a big lorry on the way, and discover Mrs Sharp tied up in the barn and all the sheep missing. They fetch Mr West again and stop the Beasleys' lorry, but find it empty with no sign of any sheep.

Joe and Jill go searching on the moors for places the Beasleys could have hidden the sheep but find no trace, so they go back to the Beasleys farm to ask Mike. Jill distracts the Beasleys while Joe sneaks into their farmhouse.

Joe can't find Mike but does see part of a message from Ted Beasley to Mr Burns the butcher, telling him to be ready for a delivery of sheep. The Beasleys decide to get rid of the stolen sheep immediately. They grab a tin of special marking paint and set off in their Land Rover. Joe jumps in the back and pours the paint onto the road, leaving a trail for Jill to follow

When the Beasleys arrive at their destination they discover Joe and lock him up in one of the huts, which is already holding Mike as a prisoner. Mike reveals that he had been stealing some sheep from Castle Farm to sell to his father, the butcher, and the Beasleys found out about it. With Jills help Joe and Mike escape from the hut.

Mike and Joe race off to Ten Bridges and arrive in time to see the Beasleys confronting Mr Burns. They confront the villains and tell them they're surrounded, but Ted is having none of it and pulls out his shotgun.

Jill and Mr West arive with some of the hostellers. Ted shoots Mr Burns in the shoulder then the Beasleys dash off into the town. Mike soon restrains Denis while Joe tackles Ted into the canal. The Beasleys and the Burnses are arrested, Joe is made foreman of the farm and all the sheep rustling is stopped