Harry Potter is spending his summer holidays with the Dursleys when he meets Dobby, a house-elf, who warns him not to go to Hogwarts. When the young wizard refuses, the house-elf drops a large pudding in the Dursleys' kitchen and Harry is punished for it. The Dursleys lock him in his room, but soon afterwards he is freed by Ron and his twin brothers and is taken to The Burrow in an enchanted flying car.
It is time to return to school. Harry and Ron try to get through the portal at King's Cross Station, but find it closed. So they steal Mr Weasley’s car and fly it to Hogwarts, only to crash into the Whomping Willow. They escape being expelled but are given detention. School days pass quickly. Inside the castle Harry is the only person who hears a mysterious voice threatening to kill. After a Halloween party, Mrs Norris the cat is found Petrified. The students are scared; everyone is talking about the Chamber of Secrets and the monster within.
During a Quidditch match, Gryffindor against Slytherin, Harry is injured by a rogue Bludger. He ends up in hospital where he is visited by Dobby again. The house-elf admits that he closed the portal at the station and bewitched the bludger. A student ends up Petrified and Harry learns that the Chamber of Secrets was opened fifty years before.
Professor Lockheart starts a Duelling Club. When Draco Malfoy sets a serpent against Harry, the wizard stops its attack by speaking in parseltongue. The students are terrified and suspect Harry of being the heir of Slytherin. The rumour intensifies when the young wizard finds another student and one of the ghosts Petrified.
Hermione brews a Polyjuice Potion so they can transform into Malfoy's friends and get their enemy to admit that he is the real heir. But the boys discover that Draco does not know who is opening the Chamber. Ron and Harry find an empty diary. Harry is curious and starts writing in it. The diary was enchanted by a boy called Tom Riddle; through it Harry sees Hagrid, fifty years earlier, accused of setting a giant spider onto a girl and killing her.
When Hermione and another girl are Petrified, Ron and Harry use an invisible cloak to get to Hagrid’s hut. Hidden behind the cloak they hear the Minister of Magic arresting Hagrid and removing Dumbledore as headmaster. Hagrid instructs them to follow the spiders. These lead the boys into the Forbidden Forest. Here Ron and Harry are captured by a colony of giant spiders. The leader, Aragog, admits that he is Hagrid’s monster. The boys find out that the monster’s victim could be Moaning Myrtle, now a wraith in the toilets.
During a visit to Hermione, Ron and Harry find a piece of paper in her hand bearing the word ‘basilisk’. Their friend had discovered the identity of the real monster. The teachers panic when Ginny Weasley is abducted, and they send the students home.
Ron and Harry decide it is time to act, so they take Professor Lockheart to Moaning Myrtle’s toilet and there they find a secret passage through one of the sinks. The boys and their hapless teacher go down the passageway. Lockheart tries to bewitch them but the curse falls on himself. Ron and Harry are separated, and Harry decides to enter the chamber alone.
The young wizard finds Ginny Weasley alive, but he also meets Tom Riddle. The latter admits that he is Lord Voldermort, the last heir of Slytherin, and he sets the basilisk onto Harry. Dumbledore’s phoenix enters the chamber and drops the Sorting Hat in Harry’s hands, whilst it punctures the basilisk’s eyes. In the Sorting Hat, Harry finds Gryffindor’s sword and with it kills the snake. The wizard destroys Tom Riddle by pushing the basilisk’s poisonous fang into the diary.
Back in Dumbledore’s office, Harry tells his story. Lucius Malfoy, furious that the headmaster is back, is accused of giving Ginny the bewitched diary. He is also tricked by Potter into freeing Dobby. The students celebrate with a midnight feast and Gryffindor win the House Cup for the second time running.