Harry Potter and the Chamber of Secrets

Harry Potter's summer holiday with the Dursleys is not going well. The Dursleys have locked away all of Harry's school possessions. Harry has not yet received any letters from his friends, Ron and Hermione. On his twelfth birthday, Dobby, the house elf, arrives to warn Harry that he will be in mortal danger if he returns to Hogwarts. Harry is determined to return, as it is the only true home he has ever known, so Dobby destroys the pudding intended for an important dinner party attended by Uncle Vernon's potential client and the client's wife. The Ministry of Magic sends a warning letter explaining Harry cannot use magic outside school and he risks expulsion if he does so again. No longer afraid of Harry's potential as a wizard, Uncle Vernon tells Harry he cannot return to Hogwarts and puts bars on Harry's window, imprisoning his nephew.

Ron and his older twin brothers Fred and George rescue Harry and take him to their home, The Burrow, in a flying car, a Ford Anglia. Ginny Weasley, Ron's younger sister, is just starting at Hogwarts and is overwhelmed when she meets Harry. After a pleasant month living at the Burrow, Harry and five of the Weasley children head to Platform 9¾ to take the Hogwarts Express back to school. Harry and Ron are shocked when they are unable to enter the barrier between platforms 9 and 10 (it is later revealed that Dobby sealed it shut to prevent Harry from returning to Hogwarts). In desperation, they steal the flying car and fly to Hogwarts. They crash into the Whomping Willow, which immediately attacks, damaging the car and snapping Ron's wand in half. The wrecked car then disappears into the Forbidden Forest. The two escape being expelled from school, despite Snape's determined attempts to enforce this.

Harry's new Defense Against the Dark Arts teacher is a famous wizard and author, Gilderoy Lockhart, who writes about his experiences with dark creatures. His books all have alliterated names, and while people admire his stories, the teachers find him to be annoying as he doesn't teach students their lessons properly, instead using lessons for self-publicity. Events at school take a turn for the worse when the legendary Chamber of Secrets is opened and a monster begins to stalk the castle, petrifying anyone who indirectly meets its gaze. According to legend, the Chamber was built by one of the Hogwarts founders, Salazar Slytherin, and can be opened only by his heir, in order to purge Hogwarts of "all those who were unworthy to study magic" or 'Muggle-borns'. Harry hears something lurking in the walls of Hogwarts, whispering its desire to kill, but no one else hears what he does. Many suspect Harry is the heir of Slytherin because, while trying to save another student during a duel against Draco Malfoy, he inadvertently speaks Parseltongue, the ability to talk to snakes and a talent that characteristically belongs to Dark Wizards and associated with Slytherin House.

Harry, Ron, and Hermione suspect Draco, Harry's rival, is the culprit, but after creating a Polyjuice Potion and sneaking into the Slytherin common room to impersonate Malfoy's cronies, Crabbe and Goyle, their suspicion proves to be wrong. Harry finds a diary that belonged to Tom Marvolo Riddle, a former student at the time the Chamber was last opened, 50 years before. When Harry writes in the diary, it writes back to him and invites him to share its memories, which shows him that Hagrid was to blame for opening the Chamber. As the school year progresses with more attacks, the Ministry arrests Hagrid, assuming he is responsible for the attacks, and suspends Dumbledore for not taking appropriate measures to stabilize the school's safety. Harry and Ron decide to take control of the situation; their motivation is to bring justice and solve the crime. That motivation is intensified by the attack on Hermione, who—by the paper still clutched in her hand when she was found—shows that the monster of Slytherin is a basilisk, and anyone who looks into its eyes will die. The reason no one has yet died is that they had not looked directly at the basilik, but only at its reflection in a puddle or mirror.

Ginny is taken directly into the Chamber, and Hogwarts is in imminent danger of closing. Professor Lockhart has previously insisted that he could take on Slytherin's monster single-handedly, and the rest of the staff tell him he now has his chance. Harry and Ron tell him that they know where the entrance of the Chamber is located and what lies within, but they find that their Defense Against the Dark Arts teacher is a fraud. He has never performed any of the feats he wrote about in his books; rather, he found those who actually performed them, got information about how they did it and then erased their memories so that he could take the credit. Harry and Ron overpower Lockhart and take him with them to the entrance of the Chamber and venture forward. During the journey through a tunnel, Lockhart grabs Ron's wand and intends to erase both of the boys' memories. This plan backfires when Ron's broken wand casts the memory charm on Lockhart himself, making him lose his own memories. Harry tells Ron to watch over Lockhart while he tries to save Ginny and tells him to come after him if he isn't back within an hour.

Harry enters the Chamber and meets Tom Marvolo Riddle (who is actually Lord Voldemort in another form: his name is an anagram of I am Lord Voldemort), who has been manipulating Ginny into, through his old diary, ordering the attacks and opening the Chamber with this enchanted diary that Harry had found. Riddle reveals himself to be Lord Voldemort in his youth, as a student at Hogwarts; he opened the Chamber fifty years ago and framed Hagrid for the crime. Harry fights against the monster, which is a basilisk, with the help of Dumbledore's pet phoenix, Fawkes, and the sword of Godric Gryffindor, another of Hogwarts' founders. After a terrifying struggle, Harry kills the basilisk with the sword, but is wounded by one of its fangs. Basilik venom being deadly poisonous, Harry is only just saved from death by the phoenix's tears, which have healing powers. Harry stabs the diary with the fang, and destroys Riddle's soul fragment and revives Ginny. Harry, Ron, Ginny and Lockhart return to the school triumphantly with Fawkes flying them there with his great strength.

Dumbledore is reinstated and Hagrid is released. Dumbledore explains to Harry his ability to speak Parseltongue; the night Lily and James Potter were killed, Voldemort inadvertently transferred some of his powers to Harry's infant body. Furthermore, the headmaster reveals that having this ability has no bearing on Harry's worthiness to be of the House of Gryffindor, rather it is the choices that define one's character and only one worthy of that house could have summoned Gryffindor's sword in that time of need. Afterward, Lucius Malfoy, Draco's malicious father, arrives at Hogwarts, followed by none other than Dobby, who is the Malfoys' house elf. Harry realizes Lucius is responsible for the attacks; before the school term began, Lucius slipped the diary, containing Voldemort's soul fragment, into Ginny's cauldron, with the hope that he could discredit Arthur Weasley's work in the Misuse of Muggle Artefacts Office of the Ministry of Magic and effect Dumbledore's removal as Headmaster of Hogwarts. Having failed to achieve these ends, Lucius leaves in a rage, but not before Harry cunningly frees Dobby by slipping the diary into his sock, which he hands to Lucius Malfoy, who in temper tosses it to Dobby. Being presented a piece of clothing, Dobby is free from his enslavement to the Malfoys.

At the conclusion, all of the basilisk's victims are revived and Gryffindor wins the House Cup.