Harry Potter and the Chamber of Secrets begins as Harry spends a miserable summer with his only remaining family, the Dursleys. During a dinner party hosted by his uncle and aunt, Harry is visited by Dobby, a house-elf. Dobby warns Harry not to return to Hogwarts, the magical school for wizards that Harry attended the previous year, explaining that terrible things will happen there. Harry politely disregards the warning, and Dobby wreaks havoc in the kitchen, infuriating the Dursleys. The Dursleys angrily imprison Harry in his room for the rest of the summer. Harry is rescued by his friend Ron Weasley and his brothers in a flying car, and spends the rest of the summer at the Weasley home.
While shopping for school supplies with the Weasleys, Harry encounters Gilderoy Lockhart, the new Defense Against the Dark Arts teacher, a wizard famous for all manner of deeds, who demands to be in a photo shoot with Harry. Harry then encounters Lucius Malfoy, a Hogwarts governor and the father of his school enemy, Draco, who gets into a fight with Ron's father when he insults the Weasley family. As Harry prepares to return to Hogwarts, he finds that he and Ron are unable to go through the secret entrance to Platform 9 ¾, so they fly the Weasley car to Hogwarts. They land messily, and both boys are given detentions.
Lockhart quickly proves to be an incompetent teacher, more concerned with students learning about his personal accomplishments. On Halloween, someone petrifies the school caretaker's cat and writes a message declaring that "The Chamber of Secrets" has been opened. Before the cat is attacked, Harry twice hears an eerie voice. He hears it first during his detention and second during a party, moments before the cat is attacked. Everybody in the school is alarmed. Harry, Ron and their other friend, Hermione Granger, learn that during the founding of Hogwarts one of the founders, Salazar Slytherin, left the school, disagreeing with the decision to teach magic to Muggle-born students. According to legend, Slytherin secretly built the Chamber of Secrets, which supposedly houses a monster only Slytherin's heir can control.
Suspecting that Draco is the heir of Slytherin, the trio start making Polyjuice Potion, a brew which allows them to take on another's form. During the school's first game of Quidditch, Harry is pursued continually by a Bludger, an enchanted ball that knocks players off their brooms, despite their purpose being to unseat as many players as possible. As a result, Harry's arm is broken, and Lockhart then proceeds to unintentionally remove the broken bones. That night, as he recovers from the injury, Harry is visited by Dobby, who admits to having orchestrated the platform incident and the rogue Bludger, both of which were attempts to keep Harry away from Hogwarts. Soon after, a first year student is attacked.
Lockhart begins a dueling club; and during the first meeting Harry terrifies his fellow students by speaking in Parseltongue to a snake. Harry's ability frightens the others because Salazar Slytherin was also able to speak Parseltongue, and his heir would also have this ability. Harry comes under further suspicion when he stumbles upon the petrified bodies of Justin Finch-Fletchley and Nearly Headless Nick.
At Christmas, Harry and Ron use the finished Polyjuice Potion to disguise themselves as Draco's friends Crabbe and Goyle. They find out that Draco is not the heir of Slytherin, but he does reveal that the Chamber was last opened fifty years ago. No more attacks occur for a while, and right before Valentine's Day, Harry finds a diary in a flooded bathroom and takes it. He writes in the diary, which responds by writing back. Through this dialogue, Harry meets Tom Riddle, a boy who many years before had accused Hagrid, the Hogwarts gamekeeper, of first opening the Chamber of Secrets. Some time later, Harry's room is ransacked and the diary is stolen.
Later on, Hermione and a Ravenclaw girl are petrified. Harry and Ron venture out of the castle to question Hagrid. Before they can question him, however, the Minister of Magic, Cornelius Fudge, takes Hagrid to Azkaban as the supposed previous culprit; while at the same time Lucius Malfoy orchestrates the removal of Hogwarts Headmaster Albus Dumbledore for his failure to stop the attacks. As Hagrid is led away, he instructs the boys to "follow the spiders", as they will be able to provide more information. Harry and Ron then sneak into the Forbidden Forest to follow the spiders. They discover the monster who killed the girl fifty years before was not a spider, that the girl's body was found in a bathroom, and that Hagrid is innocent. The boys are almost killed by the colony of giant spiders. As they escape, Harry and Ron realize that Moaning Myrtle, the ghost who haunts the bathroom where they made the Polyjuice Potion, must have been the girl killed by the monster.
A few days later, Ron and Harry discover a piece of paper with a description of a Basilisk, a giant serpent that kills all who look it directly in the eye, in Hermione's petrified hand. They deduce that the Chamber's monster is indeed a Basilisk, since as a snake it speaks Parseltongue, which Harry can understand. As for the petrifications, these were due to the victims looking at the Basilisk's eyes indirectly. Before the boys can act on their knowledge, the teachers announce that Ron's sister Ginny Weasley has been taken into the Chamber. Lockhart arrives, and is pressured by the other teachers into venturing into the Chamber and dealing with the Basilisk. Harry and Ron go to give him their information, only to discover that he is a fraud. Regardless, they force him to accompany them to the Chamber.
The trio discovers that the entrance to the Chamber is in Myrtle's bathroom, and Harry's Parseltongue is able to open it. Inside the Chamber, Lockhart steals Ron's wand, and attempts to wipe the memories of the other two, in order to keep his secrets safe. However, Ron's wand, which has been broken since the car crash at the start of the year, deflects the spell back at Lockhart, wiping his memory. A cave-in then separates him and Ron from Harry, who is forced to proceed alone.
Harry finds Ginny's unconscious body, as well as the almost-physical form of Riddle. Riddle explains that Ginny has been talking with him via his diary. Through this, Riddle was able to possess Ginny, and use her to control the Basilisk. Ginny eventually became suspicious of the diary and tried to dispose of it in a toilet, where it was picked up by Harry, but stole it back for fear Harry would find out her role in the attacks. Riddle forced her to enter the Chamber, and possessing her soul was able to obtain a physical form. Riddle reveals that he is actually Lord Voldemort, the wizard who murdered Harry's parents eleven years ago, and sets the Basilisk on Harry.
Just when it seems Harry will be killed by the Basilisk, Fawkes, Dumbledore's pet phoenix, appears and blinds the Basilisk, depriving it of its deadly stare. Fawkes also drops the school Sorting Hat, from which Harry draws a sword and uses it to kill the Basilisk. Harry then stabs the diary with a Basilisk fang, killing Riddle and saving Ginny. The five of them later leave the Chamber. Back at Hogwarts, they discover that Dumbledore has been reinstated as Headmaster.
After Harry finishes explaining things to Dumbledore, Lucius Malfoy suddenly bursts in to Dumbledore's office. It is implied that he had planted Riddle's diary on Ginny in the first place, in the hopes of discrediting Dumbledore and the Weasleys. Discovering that Mr. Malfoy is Dobby's master, Harry then tricks him into freeing Dobby by concealing a sock in the diary (clothing being the only object able to free a house elf). All the petrified people are revived by the Mandrake Draught potion, Lockhart is forced to resign whilst he regains his memories, and Hagrid returns to the school.
Harry Potter and the Chamber of Secrets is the second novel in the Harry Potter series written by J. K. Rowling. The plot follows Harry's second year at Hogwarts School of Witchcraft and Wizardry, during which a series of messages on the walls on the school's corridors warn that the "Chamber of Secrets" has been opened and that the "heir of Slytherin" will kill all pupils who do not come from all-magical families. These threats are followed by attacks which leave residents of the school "petrified" (that is, frozen). Throughout the year, Harry and his friends Ron Weasley and Hermione Granger investigate the attacks, and Harry is confronted by Lord Voldemort, who is attempting to regain full power.
