Hermione stays in the hospital wing for several weeks recovering from her cat transformation.
Ron and Harry visit her regularly.
One evening, Harry and Ron are walking back from the Hospital Wing when they hear Filch complaining.
He's furious because someone has flooded the corridor near where Mrs. Norris was attacked.
The water looks like it's coming from Moaning Myrtle's bathroom.
Harry and Ron go to investigate.
Moaning Myrtle is crying even louder than usual.
She's upset because someone through a book into her toilet: "I was just sitting in the U-bend, thinking about death, and it fell right through the top of my head" (13.29).
Harry and Ron find the book on the floor. It's a fifty-year old diary with the name of the owner – T.M. Riddle – written on the first page.
Otherwise, the book is blank.
Ron recognizes the name of the owner: "T.M. Riddle got an award for special services to the school fifty years ago" (13.39).
Harry also realizes that Riddle must have been Muggle-born: the diary comes from a store on Vauxhall Road, in London.
When Hermione finally gets out of the Hospital Wing, she examines the book for hidden enchantments.
She can't find anything, though. It really does look like the diary is empty.
Harry keeps hanging onto the diary, nevertheless.
Harry also finds out that Riddle received "an old Medal for Magical Merit" (13.71), and that he had been Head Boy of Hogwarts in his time.
The Mandrakes are quickly approaching maturity, so Professor Sprout is confident they'll be able to cure the Petrified students, ghost, and cat soon.
There's been no sign of activity from the Heir of Slytherin.
Things seem to be calming down a bit.
To improve the school's morale, Professor Lockhart makes this boneheaded plan to celebrate Valentine's Day.
In addition to dressing in pink robes and filling the Great Hall with Valentine decorations, he also brings in "a dozen surly looking dwarfs […] wearing golden wings and carrying harps" (13.87) to deliver Valentines to people.
As Harry is going to Charms class, a "particularly grim-looking dwarf" corners Harry.
Harry tries to get away, but his bag tears and he has to stop to gather his things.
The dwarf forces him to accept a singing Valentine in front of a hallway full of students.
The Valentine is so hilarious that all the students present start laughing hysterically.
Draco notices Tom Riddle's diary lying on the floor with Harry's stuff and picks it up.
Ginny "[stares] from the diary to Harry, looking terrified" (13.108).
Harry spells the diary to fly out of Draco's hands.
Draco mocks Ginny for sending Harry such an awful Valentine.
She runs off, embarrassed.
Harry notices that Riddle's diary, which got soaked in ink, remains blank and pristine.
Later that night, Harry tries writing in Riddle's diary to see what will happen.
When he writes, "My name is Harry Potter" (13.118), the diary replies, "Hello, Harry Potter. My name is Tom Riddle. How did you come by my diary?" (13.121).
Harry and Riddle speak back and forth about Hogwarts and the Chamber of Secrets.
Riddle offers to show Harry his memories of that time fifty years ago, when the Chamber was last opened and a girl died.
Harry experiences Riddle's memories as though he is standing in the room.
He sees young Tom Riddle asking Headmaster Dippet if he can stay at Hogwarts over the summer rather than returning to his "Muggle orphanage" (13.157).
Headmaster Dippet refuses, "Particularly in light of the recent tragedy…the death of that poor little girl" (13.166).
Riddle wants to know, if he can find the person who killed the girl, if Headmaster Dippet would change his mind.
Riddle walks through the halls of Hogwarts, thinking hard.
A tall wizard "with long, sweeping auburn hair and a beard" (13.176) stops Riddle in the hallway.
It's young-ish Professor Dumbledore.
He tells Riddle to hurry off to bed.
Riddle doesn't, though; instead, he goes down into the dungeons, sits down, and waits.
At last, they hear someone coming.
The person is whispering, "C'mon…gotta get yeh outta here…C'mon now…in the box" (13.186).
It's Hagrid as a schoolboy.
Riddle tells Hagrid that he can't cover for Hagrid anymore: "I don't think you meant to kill anyone. But monsters don't make good pets […] The least Hogwarts can do is make sure that the thing that killed their daughter is slaughtered" (13.194-7).
A giant spider comes rushing out of the door of the castle.
Riddle raises his wand, but Hagrid tackles him to protect the spider.
Harry wakes up from Riddle's diary.
He tells Ron, "It was Hagrid […] Hagrid opened the Chamber of Secrets fifty years ago" (13. 207).