The main idea that Henry James conveys in The Turn of the Screw does not come across as traditionally clear as the reader would wish. This is because the focalized perspective of the novel is the young governess who may, or may not, be of sound mind.
This being said, the main idea of The Turn of the Screw is the basic battle between good versus evil, where evil is represented in the ghosts of Miss Jessel and Peter Quint, and the "good" is represented in the innocent children of the household, Miles and Flora.
Throughout the novel, the unnamed Governess tries to defend the children from the presumed want of the ghosts to possess them. Yet, rather than giving us the usual "ghost story" narrative style Henry James adds high activity periods where the governess sees the ghosts, followed by periods of absolutely no activity. This is what leads the reader to question whether the ghosts are actually real or a figment of the overactive imagination of the governess.
Regardless of the addition of supernatural elements, the plot is moved forward by that essential theme of "good versus bad". This central theme is what ultimately affects every element of the novel, from the narrative style, to the stories of each of the characters, and to the final outcome of the novel itself.