Frightening messages begin appearing on the walls on the school's corridors, saying that the "Chamber of Secrets" will be re-opened. Harry and his friends are curious, and try to discover what it all means. They discover "Moaning Myrtle," the ghost of a former student who now haunts one of the girls' bathrooms. Through her, Harry discovers a diary with the name Tom Marvolo Riddle. The pages are blank, but when Harry writes in it, responses appear magically. The diary also allows him to see Hogwarts in earlier times, when Hagrid, the current groundskeeper, was a student. The diary shows Hagrid being blamed for opening the Chamber of Secrets.
Meanwhile, students whose parents are Muggles (non-magical people) are petrified, one at a time, and Hermione is one of them. Harry is more determined than ever to find the Chamber of Secrets so he can save his friend.