It happens during the chain of events surrounding Anna is abandoned by Marnie. I'm sure you'll know that it is pretty devastating in the movie, but it kind of breezes through it. In the book meanwhile, Marnie pleads for forgiveness from the window and disappears, all while Anna is wading down below in the deep and stormy marsh. In that moment, Anna's mind races through all the ways she's abandoned, all the ways no one will know about her suffering, and simply gives up and collapses straight into the water (though it's revealed she is rescued at the start of the next chapter). Made my eyes wet.