Johnny, aged 20, can't live without Fanfan, his twin sister. There is a very particular intimacy between them. They walk together everywhere, share the same room, the same tastes, the same bike, the same camping tent. But also a conflict principle sets in. Oppressed by his overbearing need, she decides to head for the south of Portugal, the country of their parents. His dreams are shattered; he feels deeply betrayed by his sister. Caught between his father who is preparing to retire to Portugal and his mother who refuses to leave France, he tries to work out his own identity. His passion for rock music gets him involved with his sister's former lover. By following this charismatic leader preaching dubious ideology, he risks losing his "gentle half" forever. Meanwhile in Portugal, she becomes disillusioned. Will they manage to grow up without each other?