During the summer holidays, Alex Deuyour learns from mother that his friend Harry Potter was caught using magic outside of school at the Dursleys. Concerned of Harry's well-being as he knows Harry's relatives hate magic, Alex takes his dog Bandit and helps his friend Ron Weasley and his brothers Fred and George rescue Harry in their father's bewitched Ford Anglia, leaving behind a note for his mother.
In Diagon Alley, Alex, Harry, the Weasleys, Hermione Granger, Kate Nguyen, and Matt Crocitto notice a book-signing by Gilderoy Lockhart, Hogwarts' new Defence Against the Dark Arts teacher. There, Alex and Harry see the former's first cousin Draco Malfoy's father, Lucius, slip a book into Ginny Weasley's cauldron. After being blocked from entering Platform Nine and Three-Quarters at King's Cross railway station, Alex, Harry, and Ron take the car to Hogwarts. There, they crash into the Whomping Willow, breaking Ron's wand, and receive detention.
In detention, Harry hears a strange voice and later finds caretaker Argus Filch's cat, Mrs. Norris, petrified beside a message written in blood: "The Chamber of Secrets has been opened, enemies of the heir... beware." One of Hogwarts' founders, Salazar Slytherin, supposedly constructed a secret Chamber containing a monster that only his heir can control, capable of purging the school of Muggle-born students. To solve this mystery, Harry, Ron, Hermione, Kate, and Matt (along with a reluctant Alex, who does not believe that Malfoy is the Heir) plan to question Malfoy, using polyjuice potion, which they brew in a bathroom haunted by Moaning Myrtle, a ghost.
During a Quidditch game, Alex and Harry's arms are broken by rogue Bludgers. Dobby visits them in the infirmary and reveals that he closed the barrier to Platform Nine and Three-Quarters and made the Bludger chase Alex and Harry to force them to leave the school. He also reveals that the Chamber had been opened in the past. When Harry communicates with a snake, the school starts to believe he is the heir. Disguised as five of Malfoy's friends, Alex, Harry, Ron, Kate, and Matt learn he is not the heir, but come to know that a Muggle-born girl died when the Chamber was last opened. Alex and Harry find an enchanted diary owned by former student Tom Riddle, who opened the Chamber and blamed Rubeus Hagrid, leading to his expulsion. When the diary is stolen and Hermione is petrified, Alex, Harry, Ron, Kate, and Matt question Hagrid (though Alex does not believe he is the Heir either due to his kind nature). Professor Dumbledore, Minister of Magic Cornelius Fudge, and Lucius arrive to take Hagrid to Azkaban and remove Dumbledore from office, but he discreetly tells the group to "follow the spiders". In the Forbidden Forest, Alex, Harry, Ron, Kate, and Matt meet Hagrid's giant pet spider, Aragog, who reveals Hagrid's innocence and provides a small clue of the Chamber's monster.
After deducing that Harry can speak to snakes and has heard a voice nobody else has heard prior to the attacks, Alex correctly deduces that the monster in the Chamber of Secrets is a Basilisk, a creature capable of killing anyone who stares into its deadly gaze and petrifies them by making them look into a reflection, but proves this theory by finding a book page which identifies the creature. The school staff learns Ginny has been taken into the Chamber, and nominate Lockhart to save her. A reluctant Alex, Harry, Ron, Kate, and Matt find Lockhart preparing to flee, exposing him as a fraud. Deducing that Myrtle was the Muggle-born girl that the basilisk killed, they find the Chamber's entrance in the bathroom she haunts. Once inside, Lockhart tries to stop Alex, Harry, Ron, Kate, and Matt by using a memory charm. However, because he seized Ron's broken wand, the spell backfires, erasing Lockhart's memory and causing a cave-in that separates Alex and Harry from Ron, Kate, Matt, and Lockhart.
Alex and Harry enter the Chamber alone and find Ginny unconscious, guarded by Riddle, who turns out to be Slytherin's heir and Voldemort's younger self, and he uses the diary to manipulate Ginny into reopening the Chamber. After Alex and Harry express their loyalty to Dumbledore, the latter's pet phoenix Fawkes arrives with the Sorting Hat, causing Riddle to summon the basilisk. Fawkes blinds the Basilisk, and the Sorting Hat produces the Sword of Gryffindor, with which Alex and Harry battle the Basilisk. After a struggle, they kill it but are poisoned by its fangs.
Despite their injuries, Alex and Kate stab the diary with the Basilisk fangs, destroying Riddle and reviving Ginny. Fawkes' tears heal Alex and Harry, who return to Hogwarts with his friends and a baffled Lockhart, earning Dumbledore's praise and Hagrid's release. Alex and Harry accuse Lucius, Dobby's master, of planting the diary in Ginny's cauldron, and tricks him into freeing Dobby. The Basilisk's victims are healed, Hermione reunites with Alex, Harry, Ron, Kate, and Matt, and Hagrid is released from Azkaban.