An orphaned brother and sister, Miles and Flora, live at Bly, a country house in England; Miles and Flora are ten and eight. They live under the guardianship of their uncle, who never appears, though his attractiveness figures in the imagination of their new Governess, who is known only by that name. Mrs. Grose is the housekeeper, and as the only other adult at Bly, becomes the Governess's confidant. These are the living characters.
There are also Miss Jessel, the previous governess at Bly, deceased; and Peter Quint, valet at Bly, also deceased. There is a character named Prologue, who sets the scene. This part may be sung by a separate singer, but as in this MLS production, is often sung by the tenor who sings Peter Quint.
The theme of The Turn of the Screw appealed strongly to Britten throughout his career: corruption and innocence, or perhaps, corruption of innocence. This theme is highlighted by its association in the libretto with a line from W.B. Yeat's poem The Second Coming: "the ceremony of innocence is drowned." This line is sung three times at the beginning of Act II by the ghostly characters, Peter Quint and Miss Jessel. Its effect is very like a conjuring.
The story could be summed up this simply: Miles and Flora have come under the evil influences of the ghosts, Peter Quint and Miss Jessel. As the new person at Bly, the Governess discovers the situation and tries to counteract it, but with disastrous consequences. But there is a great deal more to be encountered in both the story and the opera than that bald summary.
Reading the opera's libretto will take about 20 minutes and is as enjoyable in its way as reading James's original story.