Harry Potter is back at his Aunt Petunia and Uncle Vernon’s house for the summer following his first year at Hogwarts, and he is upset because he hasn’t heard from any of his friends over the summer. On Harry’s twelfth birthday, the Dursleys host a business dinner and Harry receives a visit from a house elf named Dobby in his room. Dobby warns that danger awaits Harry at Hogwarts this year and says that he should not go back—also revealing that he has been stopping Harry’s friends’ letters. When Harry refuses to promise that he will not return to Hogwarts, Dobby destroys Petunia’s pudding and ruins the dinner. The Dursleys imprison Harry in his room for three days. Harry’s friend Ron Weasley and his brothers Fred and George then rescue Harry using a flying car belonging to their father, Mr. Weasley. They take Harry and his luggage back to their home, the Burrow, where Harry spends the rest of the summer in bliss.
Harry and the Weasleys go to buy their schoolbooks for the year along with Hermione Granger and her parents, who are Muggles. At the bookstore Flourish and Blotts, they meet Gilderoy Lockhart, a famous wizard who announces that he will be the new Defense Against the Dark Arts teacher at Hogwarts. Also at the store is Lucius Malfoy, who says that the Weasleys “disgrace the name of wizard” by associating themselves with Muggles.
At the end of the summer, the Weasleys drive to King’s Cross Station with Harry to board the Hogwarts Express. But at the station, everyone is able to get through the barrier to platform nine and three quarters except for Harry and Ron. They decide to fly the car to Hogwarts, but on the way, they are seen by many Muggles. When they arrive at Hogwarts the car engine fails and they land right on the Whomping Willow, a giant tree that starts to hit back at them. Ron’s wand breaks and the car drives away from them into the Forest. Harry and Ron get in major trouble for flying the car, and are warned by Dumbledore that if they break any more rules, they will be expelled.
After Harry’s first week of class, Quidditch practice begins, but his first practice is interrupted by the Slytherins. Draco Malfoy has just joined the Slytherin team as Seeker, because his father bought the entire team new brooms. When Hermione points out that Draco bought his way onto the team, he calls her a “Mudblood.” Ron later explains that this is a derogatory term for someone who is Muggle-born, but he also says that most wizards know that blood status doesn’t mean anything about magical ability.
That evening, Harry and Ron serve detention for their arrival by flying car. Ron is sent to polish trophies, while Harry helps Lockhart answer his fan mail. But in Lockhart’s office, he hears a mysterious cold voice that it seems only he can hear, saying “Let me rip you…let me kill you.” On Halloween, Harry again hears the voice and follows it down a corridor. He is unable to find what is causing it, but he does see a puddle of water and Mrs. Norris (Filch’s cat) frozen stiff as a statue. On the wall next to her is a message: “The Chamber of Secrets has been opened. Enemies of the heir, beware.” Filch immediately blames Harry for killing Mrs. Norris, but Dumbledore explains that a second-year would not have the magic required to do it. Furthermore, he says, Mrs. Norris is not dead—she is simply petrified, and once Professor Sprout’s Mandrakes are matured, they can revive the cat.
Hermione tries to find out information about the Chamber of Secrets, and so she asks the History of Magic teacher, Professor Binns, about it. Professor Binns explains that when Hogwarts was founded, Salazar Slytherin (one of the four founders) did not want to allow Muggle-borns into the school, but the other three founders disagreed with him. Legend has it that he created a hidden Chamber that could be opened by his Heir, and that a monster lives inside that would then “purge the school” of Muggle-borns. Rumors start to circulate that Harry is Slytherin’s heir, but he, Ron, and Hermione think that it is Draco Malfoy. Hermione proposes that they use Polyjuice Potion (which can transform a person’s body into someone else’s) to ask Draco about it—but they’ll need to steal ingredients from Snape’s store.
Meanwhile, Harry has his first Quidditch game of the year, but someone bewitches a Bludger to attack Harry. Harry is able to catch the Snitch and win the game, but the Bludger breaks his arm in the process. Lockhart tries to heal the arm but instead removes Harry’s bones entirely. Harry spends the night in the hospital wing, where Dobby visits. Dobby reveals that he bewitched the platform barrier and the Bludger, because he doesn’t want Harry to be in danger. Again, he insists that Harry has to go home. Then, Dumbledore and McGonagall enter carrying Colin Creevey (a first-year who idolizes Harry), who has also been petrified.
News about Colin spreads like wildfire, and the air is thick with “rumor and suspicion.” Harry, Ron, and Hermione continue to work on the Polyjuice Potion, and during Potions class Hermione successfully steals some of the ingredients from Snape. Meanwhile, Lockhart starts a dueling club to help protect the students. At the first meeting, Harry and Draco are matched up to duel. Draco conjures a snake to try to attack Harry, but when it starts to go after a Muggle-born student named Justin Finch-Fletchley, Harry tells the snake to stop, and it does. However, he says it in snake language, and everyone thinks that he was egging on the snake. Hermione and Ron tell Harry that this is not a common gift, and that Salazar Slytherin was known for talking to snakes. The whole school will think that Harry is the Heir of Slytherin—and for all anyone knows, he could be.
The next day, Harry goes to try and explain to Justin what happened, but he overhears some Hufflepuffs talking about how he must be the Heir of Slytherin. Then, in a corridor, he discovers Justin and the ghost Nearly Headless Nick, both petrified. When McGonagall discovers Harry with them, she takes Harry to Dumbledore’s office. Harry sees the Sorting Hat and wonders whether the Hat put him in the right House, as it had thought about putting him in Slytherin the year prior. He also watches as Fawkes, Dumbledore’s phoenix, is reborn. Dumbledore tells Harry that he does not think that he petrified the students, but wonders if Harry has anything he needs to tell him. Harry says no.
Harry, Ron, and Hermione put their Polyjuice Potion plan into action on Christmas. Harry and Ron transform into Crabbe and Goyle in order to question Draco about the attacks. They discover that Draco is not the Heir of Slytherin, but they also learn that the Chamber had been opened fifty years earlier.
Soon after, Harry finds a diary that had been thrown away in a bathroom near where the first attack had occurred, which is haunted by a ghost named Moaning Myrtle. Harry discovers that by writing in the diary, he can communicate with the memory of Tom Riddle, who was a student at the school fifty years prior when the Chamber was opened last. Riddle shows Harry a memory in which he caught the person who opened the Chamber after a girl was killed by the monster. This person turns out to be Hagrid, who was trying to protect a monster within the castle. When Harry tells Ron and Hermione about this, they decide not to confront Hagrid unless another student is attacked.
One day when Harry hears the cold voice again, Hermione gets a spark of inspiration and heads off to the library to try and find out information. Later, McGonagall informs Harry and Ron that Hermione and another student were petrified. Harry and Ron decide to confront Hagrid by sneaking out to his hut at night. But before they can ask him about his involvement, Dumbledore and the Minister of Magic, Cornelius Fudge, visit Hagrid. Fudge says that with the attacks going on, he must appear to be doing something, and so he takes Hagrid away to Azkaban (the wizard prison). Then, Lucius Malfoy arrives, speaking on behalf of the school’s governors, to inform Dumbledore that he is temporarily suspended. Before Hagrid leaves, he tells the boys to “follow the spiders.”
A few days later, Harry and Ron find a trail of spiders leading into the Forbidden Forest and sneak out at night to follow it. In the forest, they meet an enormous spider named Aragog, who explains that he is not the monster that lives in the Chamber of Secrets, and that Hagrid did not open the Chamber fifty years prior. Aragog also tells them that the girl who was killed died in a bathroom. When Harry and Ron escape the spiders, they realize that the girl must have been Moaning Myrtle.
Harry and Ron then visit Hermione’s body and realize that in her frozen hand, there is a piece of paper explaining that the monster in the Chamber is a Basilisk. But before they can tell the teachers, McGonagall announces that the school is closing, because the monster has taken Ginny Weasley into the Chamber. The teachers decide to send Lockhart to face the monster. Lockhart tries to run away, but Harry and Ron threaten to reveal that he is a fraud, and so he goes along with them. They discover a secret tunnel in Myrtle’s bathroom that leads them to the Chamber.
In the Chamber, Lockhart tries to perform a memory charm on Harry and Ron using Ron’s broken wand, but it backfires, causing Lockhart to lose his memory and accidentally creating a rock barrier between Ron and Harry. Harry goes on to the Chamber and discovers Ginny’s unconscious body and Tom Riddle. Riddle explains that he is actually Voldemort, and that he has been possessing Ginny through his diary and using her to carry out the attacks. Harry calls out to Dumbledore for help, and Fawkes arrives. Fawkes blinds the basilisk and delivers the Sword of Gryffindor, which Harry uses to kill the basilisk. Harry then uses the basilisk’s fang to pierce Riddle’s diary, destroying Riddle. Ginny regains consciousness, and Fawkes carries Harry, Ginny, Ron, and Lockhart out of the Chamber.
Dumbledore returns to the school, and Harry explains what happened. Harry expresses his insecurity that he and Riddle are very similar, but Dumbledore tells Harry that they make very different choices, and therefore are very different from each other. Then Lucius Malfoy arrives with Dobby to confront Dumbledore, and Harry realizes that Lucius gave Ginny the diary in Flourish and Blotts. Harry then tricks Lucius into freeing Dobby.
At the end of the book, the petrified students are cured, exams are cancelled, and Hagrid returns from Azkaban. Lockhart is fired now that he has no memory, and Harry returns to Privet Drive once more.
