Bess, a Welsh Collie sheepdog is old and ailing. Jack, her owner, has decided it is time to put her down. His young son, Henry, in trying to prevent this, causes the gun to go off injuring Jack. Believing that he is responsible for his father's death, Henry runs away, taking Bess with him. Thus begins what could turn out to be the last day of Henry's young life.

The setting is a hill farm on the border between England and Wales on a summer day in 1947. The story follows Henry's increasingly desperate attempts to evade capture and the inept and amateurish efforts of family and neighbours to find him and explain that his father's injuries are not life threatening and that no-one holds him responsible.

Each member of the family has his or her own fears and worries for the future and these influence their behaviour throughout the long sultry day as storm clouds gather. Henry's older brother, Cecil, wants to have a bigger say in management of the farm; like his mother, his sister Margery is preoccupied with planning her forthcoming marriage. His aunts too have problems that demand attention.

Assorted professionals have their own distorted view of the family and of Henry: the family Doctor, the district nurse, the vicar, the school teacher. Henry meanwhile faces an increasing number of set-backs and injuries which lead him to make a near fatal decision. Rescue finally comes from a surprising quarter.