Taken at face value, The Turn of the Screw is about the classic struggle between good and evil: you've got your obvious bad guy, who also happens to be a creepy ghost-stalker. On the other hand, you've got your good guy, or rather, good gal in this case, who's defending a pair of innocent children from the grasp of the scary ghost. However, once you look below the surface, the issue gets a little more confusing. Depending on how you read the story, different characters can get assigned to the sides of good and evil…or one might decide that the black and white standards of good and evil don't even apply.
At its core, The Turn of the Screw is fundamentally a story about the struggle between good and evil.
"Good" and "evil" are eventually discarded by the end of this book; the growing ambiguity of all of the characters makes it impossible to continue to define any of them as such.