Heir of Slytherin
Events at school take a turn for the worse when the legendary Chamber of Secrets is opened and a monster stalks the castle, petrifying anyone who meets its gaze. According to legend, the Chamber was built by Salazar Slytherin and can only be opened by his heir, in order to purge Hogwarts of "all those who were unworthy to study magic" - i.e. Muggle-born wizards. Many suspect Harry is the heir of Slytherin, when he inadvertently speaks Parseltongue, a rare talent which he gained during Voldemort's unsuccessful attack on him eleven years previously. Harry himself begins to believe he might be Slytherin's Heir, as the Sorting Hat wanted to put him in Slytherin when he entered Hogwarts.
In order to discover the identity of Slytherin's Heir, Ron and Harry use
Polyjuice Potion to disguise themselves as Slytherin students, Crabbe and Goyle, as they think Draco Malfoy is the Heir. He isn't, but he does inadvertently provide Harry and Ron with an important clue about the Chamber of Secrets. Later, Harry finds a blank diary belonging to Tom Riddle and decides to keep it.
The attacks increase throughout the year, petrifying more students, including Hermione. All activities, including
Quidditch, are cancelled, and students are not allowed to leave their dormitories or classes without their teachers. A message written on a wall declares that Ginny Weasley has been taken into the Chamber, where "her skeleton will lie forever."
Harry discovers the location of the entrance to the Chamber of Secrets. He and Ron force
Gilderoy Lockhart to go with them. Once they find the entrance to the Chamber, Lockhart reveals that he is not the hero he pretends to be. He attempts to use Ron's broken wand to erase Harry and Ron's memories, but the spell backfires on himself. The ceiling caves in, separating Harry from Ron and Lockhart, and it is Lockhart whose memory is erased rather than Ron's and Harry's.
Harry reaches the Chamber of Secrets, where he finds an unconscious
Ginny and a ghostlike young man named Tom Riddle, who claims to be a "memory". Harry learns that Ginny, under the control of Lord Voldemort, opened the Chamber. Voldemort, whose real name is Tom Marvolo Riddle, imprinted his memory in an enchanted diary, in order to continue the work he began when he opened the Chamber fifty years ago — ridding Hogwarts of non-pureblood witches and wizards. Hagrid, a Hogwarts student at the time, was blamed for the attacks and expelled.
It is this fragment of soul in the diary which has set loose the
basilisk, the monster responsible for petrifying the students. The basilisk attacks Harry, despite his ability to speak Parseltongue; Dumbledore's phoenix, Fawkes, arrives in the nick of time with the Sorting Hat, from which Harry draws out the mystic sword of Godric Gryffindor. Fawkes blinds the basilisk, destroying its fatal gaze, and Harry slays it with the sword. Harry's arm is pierced by the creature's poisonous fang as he kills it, but Fawkes heals Harry with his tears. Harry destroys the diary by stabbing it with one of the basilisk's fangs.