"Boy" tells the story of the Reimer family. It begins in 1966 in the USA: twin brothers Brian and Bruce are brought to a hospital by their parents for circumcision when they are babies. But this simple surgical procedure goes horribly wrong for baby Bruce. After a desperate odyssey through every conceivable medical facility, the parents finally come across an expert who advises them to let their child grow up as a girl. All they need are hormones and clothes. Just like that, Bruce becomes Brenda. Brenda knows nothing about her past, but even at a young age she senses that something is wrong...