Nearly Headlless Nick and Justin get petrified when Harry hears the strange voice again. Harry, Ron, and Hermione research about the Chamber of Secrets and come to realize that is must have been opened to cause all the kids being petrified. They start to think Malfoy is behind everything so they start to brew Polyjuice potion in the abandoned girls bathroom. Moaning Myrtle haunts that bathroom and distracts them.
Harry realizes that Malfoy is not the heir of Slytherin. He finds a diary in moaning Myrtles bathroom that belongs to Tom Riddle. When Harry writes in the diary, it responds and Tom shows Harry through his time at Hogwarts revealing that Hagrid opened the chamber of secrets years ago. While Hermione and a Ravenclaw girl are petrified in the library.
Hagrid is taken away to Azkaban for thought to have opened the chamber again. He leads a path of spiders for Harry and Ron that take them to the forbidden forest, saying that it will lead them to the chamber monster. They met up with Aragog the spider who Hagrid took care of that was claimed to be the monster but it wasn't.
Ron and Harry find a piece of parchment in Hermiones hand while she is petrified. It has the description of a Basilisk. They realize that that was the monster lurking in the chamber and petrifying everyone. They hear news that Ginny Weasley was taken into the chamber.
Harry, Ron and Lockhart go down into the chamber of secrets. Lockhart accidently curses himself by losing his memory. Harry encounters Ginny's still body and Tom Riddle. Tom turns out to be the younger Voldemort and he had been enchanting Ginny through his journal. harry calls for Dumbledore when a phoenix and the sorting hat appear before him. Tom summons a Basilisk but the phoenix punctures its eyes. The hat produces a sword that Harry uses to kill the basilisk.
Harry sticks a basilisk fang through the diary, destroying Tom. Ginny wakes up, and Harry explains his adventure to Dumbledore. Lucius Malfoy storms into his office with his houself Dobby, and Harry frees Dobby by tricking Lucius into giving Dobby a sock. All is well in the castle and the students leave for vacation.