Neal, a retarded teenager, lives with his parents, who are finding it increasingly difficult to deal with his disruptive behaviour, which is causing friction in their marriage. His only friend is Leslie, an inventive, unconventional 13-year-old girl, who devises exciting games he can join in and regales him with wondrous stories about Mars and the mystical Crystal Mountain. When Leslie intercepts a telephone call confirming there is a place for Neal at a special retardation centre, the boy is very upset and the youngsters decide to run away. They make their way to the nearby zoo, where they continue to elude the adults until Leslie diverts their attention to allow Neal to escape. The startling final scene finds him in a wild, rocky landscape, where he joins a group of children trekking towards the Crystal Mountain of his imagination.