(from wikipedia.ja):

Simon was taken in by his uncle, Gilles, an ecologist, after his parents died, but he ended up fighting with his rival Jean-Claude over his cousin Joca, his uncle's daughter.

Joca, worried about Simon's weak fighting spirit, gives him a medicine developed by his father that makes him stronger, but it turns out to be a sex-transforming drug. Simon is placed in the care of his uncle's friend in England, and when his uncle shows Simon his own chromosomes, Simon is forced to accept. A month later, Simon was pronounced dead in England.

Seven years later, after the death of his mother-in-law, Simon, now named Solange, was taken in again by his uncle and sent to live with Joca. Jean-Claude, who is engaged to Joca, becomes disturbed by Solange as a woman.