After the governess asks Miles whether or not he had stolen the letter she’d meant to have sent to his uncle, she notices through the room’s window the face of Quint, staring threateningly into the room. The governess reacts to this with another surge of her sense of protective purpose, and she decides to try to prevent Miles from seeing Quint outside.
Quint’s appearance at this moment continues a pattern that emerges throughout this story: the ghosts tend to appear when the governess is at the edge of a discovery about the children, in this case, the truth about whether or not Miles stole the letter. That may be because such moments "summon" the ghosts, or because the governess's own imagination acts up in the moment's she feels are significant.
Miles then confesses that he had stolen the letter. He says he took it to see what she had said about him. The governess says it is clear to her that Miles knows he is in the presence of something at this moment, but it is equally clear that he does not know what. After mentioning this, the governess looks at the window again, and Quint is gone.
The governess’s inclination that Miles feels but does not see the presence of Quint shows that her impression of Miles’s relationship with Quint continues to change. Instead of seeing him as deliberately secretive about the relationship, she sees him now, once more, as the innocent victim.
The governess then asks what Miles found when he read the letter. He says he found nothing, and that he burned the letter after reading it. The governess asks Miles if he stole letters at school, and she tells him she knew all along that he had been expelled. He tells the governess he was expelled for having “said things”, though he does not clarify what he had said. His confusing explanation gets stranger still when he tells her he’d said these things only to people he liked, and in his letters home. The governess starts to wonder if he had been innocent all along, and she worries what that would say about her.
The governess’s pause to reflect upon whether or not she had misread Miles expulsion provides more evidence that she’s not sure whether Miles ever was the devious person she had, for a time, thought he was. Her pause for self-reflection, in which she wonders what his innocence might say about her, suggests that she may not have the level of confidence in her shrewdness as she’s claimed to have.
Suddenly, Quint reappears at the window. The governess latches onto Miles, and yells at Quint. Miles then asks “Is she here?” and the governess infers that he means Miss Jessel. The governess responds that it is not Miss Jessel, so Miles asks if she means “he”, and the governess asks whom he means by “he.” Miles yells out “Peter Quint—you devil!” and asks where he is. The governess exclaims that Miles doesn’t need to worry, that he now is in her care. Miles looks out the window, and sees nothing. The governess says that because he now knows what he has lost, he utters “the cry of a creature hurled over the abyss”, his heart stops, and he dies.
The sudden appearance of Quint may be the ghost coming to exact revenge. But the ghost appearing at this moment, when the governess is just starting to doubt herself, could also be taken as her subconscious "saving" her from having to entertain such doubts. Miles’s death is the climax of this story. The governess believes Miles dies because of his despair at having lost Quint forever. But Miles never says he saw Quint, he only shouts his name, so it could be the case that he has died for some other reason, perhaps by being smothered by the governess’s clinging hug. Or perhaps Quint did abuse Miles in some way, and the governess by imagining the ghost of Quint has scared Miles to death. It is unclear, though, as it has been throughout—the book does not resolve itself to affirm or deny the governess’s understanding of the story or reliability as a storyteller