Hermione Granger: Professor... I was wondering if you could tell us about the Chamber of Secrets.

Minerva McGonagall: Very well. You all know, of course... that Hogwarts was founded over a thousand years ago... by the four greatest witches and wizards of the age: Godric Gryffindor, Helga Hufflepuff... Rowena Ravenclaw and Salazar Slytherin. Now, three of the founders coexisted quite harmoniously. One did not.

Ron Weasley (to Harry): Three guesses who.

Minerva McGonagall: Salazar Slytherin wished to be more selective... about the students admitted to Hogwarts. He believed magical learning should be kept within all-magic families. In other words, pure-bloods. Unable to sway the others, he decided to leave the school. Now, according to legend... Slytherin had built a hidden chamber in this castle... known as the Chamber of Secrets. Though, shortly before departing, he sealed it... until that time when his own true Heir returned to the school. The Heir alone... would be able to open the Chamber... and unleash the horror within, and by so doing... purge the school of all those who... in Slytherin's view, were unworthy to study magic.

Hermione Granger: Muggle-borns.

Minerva McGonagall: Well. Naturally, the school has been searched many times. No such chamber has been found.

Hermione Granger: Professor? What exactly does legend tell us lies within the Chamber?

Minerva McGonagall: The Chamber is said to be home to something... that only the Heir of Slytherin can control. It is said to be the home... of a monster.