The film focuses on Billy Casper, who has little hope in life beyond becoming a coal miner and is bullied both at home, by his physically and verbally abusive half-brother, Jud, as well as at school. He is mischievous himself; he steals milk from milk floats, gets other students into trouble and generally fights and misbehaves. Billy comes over as an emotionally neglected boy with little self-respect. His mother refers to him in the film as a "hopeless case".
Outside cadging money and day-dreaming at school, Billy has no positive interests. His greatest fear is ending up working down the pit as a coal miner but he has no apparent escape route from what would ultimately be his fate. That is until he finds an outlet from his pitiful existence through training a kestrel that he takes from a nest on a farm. His interest in learning falconry prompts Billy to steal a book on the subject from a secondhand book shop as he cannot get a borrower's card for the Public Library.
As the relationship between Billy and "Kes", the kestrel, during the training improves so does Billy's outlook and horizons. For the first time in the film Billy receives praise, from his English teacher after delivering an impromptu talk on his relationship with the bird.
However, when Jud sends Billy off to place a bet on a horse, Billy spends the money on chips, as he assumes the horse is unlikely to win. However, the horse does win. Jud is furious at Billy, and takes revenge by killing his kestrel because he could not find Billy.