A flashback begins. Judy's voiceover explains Gavin Elster's plan to murder his wife. Scottie, she says, was this plan's "victim," and she was its "tool."
Elster hired Judy to play Madeleine because she looked like her. He made Scottie the witness, knowing that his fear of heights would prevent him from reaching the top of the bell-tower at the decisive moment when Judy, as Madeleine, would throw herself off the tower.
It was the real Madeleine's body that fell.
Judy says that the only complication in Elster's plan was that she fell in love with Scottie.
She still adores him and wishes he could love her as Judy.
Alright, at this point we're thinking that we have to go back and reinterpret everything we've seen up to this point in the movie. Thanks, Hitchcock.