A trotter, named Jolly Roger, keeps breaking stride in the homestretch and starts to gallop. He is eventually barred from racing as a threat to the other horses and drivers. Eventually a cop purchases Jolly Roger for the Boston Mounted Police Force and through rigorous training finally schools the horse. While he is on duty at a local harness racing track (Foxboro), he gets summoned to assist a driver who has had an accident during a race. Jolly Roger breaks into a trot, swiftly races down the track, and saves the driver. Eventually his original trainer recognizes him and buys him back. From there he goes on to become a champion.