Chapter 17 - "The Heir of Slytherin" - In which Harry finds an unconscious Ginny, meets Tom Riddle (who has had Ginny in his power through the diary), and hears his story. Fawkes the phoenix arrives with the Sorting Hat. Riddle (Voldemort) calls the basilisk to kill Harry, but Fawkes intervenes and together they kill the basilisk, and by driving a serpent fang through the diary, get rid of Riddle as well.
Journey to the Chamber of Secrets
Harry and Ron find Lockhart in his office, packing to run away, and he admits all of his stories were lies. When he tries a [Memory Charm], though, Harry [disarms] him and makes him go with them into the Chamber of Secrets.
Arriving in Moaning Myrtle's bathroom with Lockhart in tow, Harry and Ron ask Myrtle. She gets excited and explains the eyes, and more importantly, where she saw them. Harry uses his Parseltongue then to open the Chamber of Secrets, and after sending Lockhart in, he and Ron follow.
Harry leads the way into the Chamber of Secrets, but Lockhart quickly grabs Ron's wand to try another [Memory Charm]. The broken wand backfires, though, trapping Harry farther in the chamber and Ron with a memory-less Lockhart further back, and Harry goes on alone.
Harry Defeats a Snake and a Riddle
Now in the Chamber of Secrets, Harry finds Ginny's body, along with Tom Riddle. Riddle explains his actions and his plans, along with the fact that he is Voldemort. He laughs when Fawkes brings Harry the Sorting Hat, and prepares to call the [basilisk] to destroy Harry.
The [basilisk] arrives and goes for Harry, but Fawkes strikes first, poking out the monster's eyes. Harry, surprised, pulls a sword out of the Sorting Hat and uses it to kill the [basilisk] - though not before the [basilisk] bites his arm, lethally poisoning him.
Fawkes begins to cry and lets his tears fall onto Harry's injured arm, and the [basilisk]'s venom, and the pain, leave him. Left one-on-one with Tom Riddle, Harry jams the [basilisk] fang into the diary, and Riddle is destroyed, awakening Ginny.
Fawkes Lends a Tail