In preparation for a visit from Uncle Vernon Dursley's client, the Dursleys banish Harry in his room. There, Harry finds Dobby the house elf, who warns against returning to Hogwarts. When Harry refuses, Dobby frames Harry for ruining Vernon's meeting. Furious, Vernon locks Harry in his room to prevent his return to Hogwarts. Ron, Fred, and George Weasley arrive in their flying car to rescue Harry and take him back to The Burrow, their home. Harry meets Ron's younger sister, Ginny, who is about to begin at Hogwarts and has developed a crush on Harry. Harry also meets Ron's father, Arthur Weasley; he had met Mrs. Weasley the previous year. Soon after, Harry and the Weasleys travel to Diagon Alley by Floo Powder. Harry accidentally mispronounces "Diagon Alley", and lands in Knockturn Alley. Fortunately, Hagrid happens upon him and reunites him with the Weasleys and Hermione. While shopping, Harry meets Gilderoy Lockhart, a famous wizard and author, and later Draco Malfoy and his father, Lucius, who praise Voldemort and deride Harry, Hermione, and the Weasleys.
At King's Cross Station, though the rest of the Weasleys reach Platform 9 3/4 without trouble, Harry and Ron find the magical barrier blocked (Dobby sealed it to prevent Harry's return to Hogwarts); as a result, they miss the Hogwarts Express. Harry and Ron manage to reach Hogwarts with the flying car, but accidentally land in the school's Whomping Willow. Ron's wand is broken and the car begins to act erratically, ejecting the boys and their luggage and driving itself off into the Forbidden Forest.
Shortly after the start of term, Harry begins to hear an ominous voice that no one else can hear. Then, Harry, Ron, and Hermione find the message The Chamber of Secrets has been opened. Enemies of the heir, beware written in blood across a castle wall, and discover that caretaker Argus Filch's cat has been petrified. Legend has it that the Chamber of Secrets can be opened only by the Heir of Slytherin; it is said to be the home of a monstrous creature that will obey only the Heir. Harry suspects the Heir is Malfoy. To interrogate him, the three brew Polyjuice Potion to disguise themselves as Malfoy's friends Crabbe and Goyle. They learn that Malfoy is not the Heir, and that he does not know who is.
Gilderoy Lockhart, hired to teach Defense Against the Dark Arts, offers a dueling club. At the meeting, Draco conjures a snake, which Harry finds he can talk to. Hermione explains that he is a Parselmouth like Slytherin, a connection that causes the school to believe Harry is the Heir and ostracise him.
In a bathroom, Harry finds a book with nothing written in it that once belonged to someone named Tom Marvolo Riddle. Through the book, Harry sees events that happened fifty years ago, when Tom was a student. Tom's memories incriminate Hagrid as the Heir.
Over the course of the school year, several more students (Colin Creevey, Justin Finch-Fletchley, and Hermione) and even the Gryffindor ghost Sir Nicholas are found petrified and Tom Riddle's diary goes missing. Harry and Ron decide to see Hagrid at his hut, but before they can speak to him Cornelius Fudge and Lucius Malfoy arrive. While Ron and Harry hide, the visitors tell Hagrid they are suspending Dumbledore as headmaster and arresting Hagrid, under suspicion of having opened the chamber. Before Hagrid is taken away, he tells Ron and Harry to follow the spiders into the Forbidden Forest for the truth. They do so and meet Aragog, a giant spider thought to have killed a student fifty years ago. Aragog reveals that he is not the monster who killed the student, and that Hagrid is innocent.
Harry and Ron find out from a piece of paper in Hermione's hand that the monster responsible for the attacks is a Basilisk. They also find another message from the Heir and the teachers say that Ginny has been taken into the chamber. Lockhart is sent to find the chamber and save Ginny, but he tries to make an escape until Harry and Ron catch him. It turns out Lockhart's famous past is false; he used memory-erasing charms on witches and wizards so that he could take credit for their great accomplishments. The three find the chamber entrance in Moaning Myrtle's bathroom and enter to find a giant snake skin. Lockhart tries to stop Harry and Ron by using a memory charm, but it backfires because he is using Ron's broken wand. Lockhart loses his memory and part of the chamber caves in, separating Harry from the others. Harry finds Ginny's body, and Tom Riddle appears, explaining that he is a memory preserved in the diary for fifty years. He goes on to tell Harry that he is taking over Ginny's soul, so that he may regain power. Harry learns that Tom is Slytherin's Heir and is actually Lord Voldemort in his teenage form. Riddle sends the Basilisk to kill Harry but Dumbledore's phoenix, Fawkes, attacks the Basilisk's eyes. Fawkes gives Harry the Sorting Hat, in which appears the Sword of Godric Gryffindor. After a terrifying battle, Harry impales the Basilisk in the roof of the mouth, killing it. Unfortunately, a fang pierces Harry's arm.
Dying from the fang's poison, Harry destroys the memory of Tom Riddle by piercing the diary with the fang retrieved from his arm. Ginny regains consciousness and finds Harry hurt, but Fawkes heals Harry's wound with his tears. Dumbledore is returned to the school and Hagrid is released from Azkaban. Harry, learning that Dobby serves the Malfoys, tricks Lucius into freeing him from servitude. Everyone who has been petrified is finally restored, using a potion that had taken months to develop.
