On July 31, 1992, Alex Deuyour learns from his mother, Desiree, that the Ministry of Magic detected Harry using magic at the Dursleys. Concerned, he sends a letter to his friends Ron Weasley, Hermione Granger, Kate Nguyen, and Matt Crocitto, who has decided to join the group. Alex joins Ron and his brothers, twins Fred and George, to rescue Harry from the abusive Dursleys using Ron's father, Arthur's bewitched Ford Anglia. They take him to the Weasleys' home, the Burrow. Alex, Harry, and the Weasley family travel to Diagon Alley for school supplies. They run into Hermione and Kate and meet Lucius Malfoy, father of Draco, and also Gilderoy Lockhart, a conceited autobiographer and adventurer who is the new Defence Against the Dark Arts professor. At King's Cross station, Alex, Harry, and Ron enter Platform 9 and miss the Hogwarts Express. They fly in Mr Weasley's car to Hogwarts, crashing into the Whomping Willow on school grounds and damaging Ron's wand. The car then escapes into the forest.
Later, Harry is the only one who hears a strange voice emanating from the castle walls. Soon after, Mr Filch's cat, Mrs Norris, is found petrified, along with a bloody warning scrawled on a wall: "The Chamber of Secrets has been opened. Enemies of the heir, beware". Salazar Slytherin, one of the school's founders, created the Chamber after a dispute with fellow founders on admitting Muggle-born students. Some in the wizarding community disdain Muggle-born wizards like Hermione, believing pure-bloods are superior. The Chamber supposedly houses a monster that only the Heir of Slytherin can control.
During a Quidditch game, a rogue Bludger strikes Alex and Harry, breaking their arms. Lockhart blunders an attempt to repair them, sending Alex and Harry to the hospital overnight. Dobby, a house-elf that was responsible for Harry being accused of using magic at the Dursleys, visits Harry and Alex and reveals he jinxed the Bludger and sealed the portal at King's Cross. He says the Chamber of Secrets was once opened years before. Dobby also tells them that house-elves like himself serve one master their entire life, and cannot be freed unless given proper clothing. After another attack, students attend a defensive duelling class, during which Harry exhibits a rare ability to speak "Parseltongue", the language of snakes.
Harry, Ron, Hermione, Kate, and Matt suspect Draco is the Heir, given his hostility toward Muggle-borns. Alex however points out that Draco has to be the heir of Salazar Slytherin and says that the Malfoys are not related to Salazar Slytherin. Despite this, Hermione brews Polyjuice Potion, allowing Alex, Harry, Ron, Kate, and Matt to impersonates Draco's friends, Blaise Zabini, Goyle, Crabbe, Pansy Parkinson, and Theodore Nott. They learn that Draco knows nothing about the heir. Meanwhile, Moaning Myrtle, a ghost that haunts a girls' bathroom, shows the group a diary left in her stall. It belonged to Tom Riddle, a student who witnessed another student's death during the Chamber's previous opening. Riddle's consciousness within the diary claims that Hagrid was responsible. Hermione is petrified in the next attack. The school is put on lockdown and may close. Headmaster Albus Dumbledore is forced out and Hagrid is sent to Azkaban prison.
Following instructions left by Hagrid, Alex, Harry, Ron, Kate, and Matt follow spiders into the Forbidden Forest. They encounter Aragog, a gigantic Acromantula which denies its involvement, and claims spiders fear the real monster. Aragog attempts to feed Alex, Harry, Ron, Kate, and Matt to its progeny, but Mr Weasley's car rescues them. Alex correctly deduces that due to Harry speaking Parseltongue and hearing it before the attacks, the monster is a Basilisk, a giant snake whose direct gaze kills and petrifies victim when seen indirectly through a secondary reflection. Alex is proven correct when Alex, Harry, Ron, Kate, and Matt discover that Hermione deduced this as well. The basilisk is the voice in the walls, and it travels through the plumbing. Moaning Myrtle was the student that was killed.
Ron's sister Ginny is abducted and taken into the Chamber. Harry, Ron, Kate, and Matt discover the entrance in Myrtle's bathroom, and much to Alex's dismay (as he correctly deduces that Lockhart was a fraud), force Lockhart to enter it with them. Lockhart confesses he is a fraud who made up all of his stories and attempts to erase the group's memory after stealing Ron's damaged wand. The spell backfires, obliterating his own memory, and causes a cave-in; Ron, Kate, and Matt are separated from Alex and Harry and the former three stay behind to help Lockhart.
Alex and Harry proceed to the Chamber and find an unconscious Ginny. A manifestation of Tom Riddle reveals he is Lord Voldemort and the Heir of Slytherin, which Alex correctly deduced after noting his fear in the flashback, something Harry did not notice. He opened the Chamber and framed Hagrid. He has been using the diary to possess and control Ginny, who had been behaving strangely. The basilisk appears to kill Alex and Harry. Dumbledore's phoenix Fawkes arrives, bringing Alex and Harry the Sorting Hat. Fawkes blinds the basilisk and Alex and Harry pull the Sword of Godric Gryffindor from the Sorting Hat. They slay the Basilisk but are poisoned by its venom. As Riddle taunts the dying Alex and Harry, Fawkes' tears heal them. Harry and Alex stab Riddle's diary with a severed Basilisk fang, destroying it and Riddle's body, and reviving Ginny.
Alex, Harry, Ron, Kate, Matt, Ginny, and Lockhart return to the castle. Alex and Harry give the diary to Dumbledore. Lucius bursts in, furious that Dumbledore returned. He is accompanied by Dobby, who is the Malfoys' house-elf and was working to protect Alex and Harry. Alex points out that Lucius slipped the diary into Ginny's cauldron while they were in Diagon Alley to open the Chamber. Alex and Harry trick Lucius into freeing Dobby from servitude by giving him their socks; Lucius attempts to attack Alex and Harry in revenge, but Dobby repels him.
The petrified students are cured, Gryffindor wins the House Cup again, Hagrid is released, Lockhart is confined to St. Mungo's Hospital, and Alex is happy to share what happened that school year with his mother.