In Transfiguration class with Professor McGonagall, the second-years are changing animals into water goblets when Hermione asks Professor McGonagall if she'll tell the class all about the Chamber of Secrets.
Well, it all goes back to when Hogwarts was founded over a thousand years ago. There were four founders—Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin.
Three of these guys got along fine…but one them didn't. Guess who couldn't play nice with others?
Slytherin wanted to teach only students who came from pure-blood families. So he left the Hogwarts but, according to legend, he built a secret chamber before he booked out of the castle.
His plan was for his heir to come back later and open the chamber and unleash horrors within on unsuspecting students with non-magical blood. Creepy.
Professor McGonagall tells her students that the castle has been thoroughly searched and no one has ever found this hidden chamber. So of course it doesn't exist.
But, like, just hypothetically, what would be in the chamber, Hermione asks. If it did exist?
A hideous monster, of course. Something only the heir of Slytherin could control, Professor McGonagall explains. Lovely.
After class, Harry, Ron, and Hermione discuss. If the Chamber of Secrets is real and it's been opened, that means the heir of Slytherin must be at Hogwarts.
Ron suspects Malfoy. After all, he's a pure-blood and he hates Muggle-borns, right?
Hermione's doubtful. That's pretty grand for Draco Malfoy.
Maybe they could ask Malfoy's friends, Crabbe and Goyle? Ron suggests.