For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The short-lived idyll of the Governess's new life at Bly ends abruptly with the unsettling first two appearances of Peter Quint – we know that something's very wrong when Mrs. Grose reveals that Quint is actually dead.
Act II
Though there are various moments of despair in this story, the first and most significant is the Governess's revelation that Flora is involved with the ghosts (provoked by her first glimpse of Miss Jessel at the lake); here, she worries that it's too late to save the children, and that they're already lost.
Act III
The third act begins with the Governess's suspicion of the children, and builds up towards the super-dramatic ending – Miles's death.