Every day 12-year-old Lisa takes an imaginary space trip with her little brother Paul. He’s not well and stuck at home, but the interplanetary fun is threatened by his sister’s life outdoors, especially when she meets a boy at school who might just be her first love.
Paul is the titular 'moonlit' kid. An incurable skin disease means that he is not allowed to expose himself to sunlight. This not only dominates the life of the six-year-old, but also that of his family, especially his sister Lisa, who is twice his age. The two spend every day together and experience adventures in their imagination. Paul wanders through space as a missing space captain, Lisa represents the scientist who accompanies him.
Lisa's life is further complicated by Simon, with whom the girl falls in love. Paul notices that his sister is no longer giving her attention to him alone. At the same time, the boy's condition worsens, so that Lisa feels overwhelmed to combine her first love with her concern for her brother.