Vertigo (movie)
Judy has a flashback. She is really the Madeleine Scottie knew. She was involved in a murder plot. The plan was to fake the suicide of the real Madeleine. Gavin knew that Scottie had a fear of heights. So Gavin knew Scottie would not go up the tower. As a result, Scottie wouldn't see Gavin throwing down his wife's body from the tower. Judy writes a letter telling Scottie about the murder plot but rips it up.