Hermione has been in the hospital wing with a cat-face since Christmas, but she is now getting better at last and will soon be released. After visiting her, Harry and Ron hear Filch storming off from the girls' bathroom in a fury, and they head over there to find that [[MakingASplash Moaning Myrtle has flooded the place]] because someone threw a book at her head.

The book turns out to be an old diary from a shop in London, with the name T.M. Riddle inscribed on it. Surprisingly, the diary's pages are completely empty, and no spell of Hermione's is able to reveal anything. They do, however, figure out from the trophy room that Riddle won a Special Award for Services to the School. Ron suggests that he got the award [[{{Foreshadowing}} for murdering Moaning Myrtle.]]

On Valentine's Day, Lockhart [[CrapsaccharineWorld decorates the school just for the occasion,]] and manages to severely embarrass several teachers by proclaiming their apparent abilities to perfect love aids. Ugly dwarves dressed as cupids are sent around delivering valentines; one manages to knock down Harry and his bag, shattering the ink bottle inside, before delivering a truly cringe-worthy valentine song, courtesy of [[ShrinkingViolet Ginny Weasley.]]

That night, Harry realizes that the diary, alone of all the books in his bag, avoided being covered in red ink. He opens the diary to a random page, drops some ink on it, and [[WhamMoment it disappears into the page.]] Harry then writes, "My name is Harry Potter," and the ink reforms into, "Hello, Harry Potter. My name is Tom Riddle. How did you come by my diary?" After a brief series of exchanges like this, Riddle offers to take Harry ''into his diary,'' by which the pages flip to June 13th and Harry is suddenly thrown into the diary itself, into a vision of that day 50 years ago while the Chamber of Secrets had been open. Riddle tells the headmaster (then an old man named Professor Dippet) that perhaps if someone found the person who is responsible for the attacks, that Hogwarts could be saved from closing, but he is evasive when asked if he knows anything.

After this, Riddle travels down to the dungeons, and confronts none other than a younger version of [[GentleGiant Hagrid,]] who is trying to stuff something suspicious into his trunk. Riddle tells him that he will have to report him for his possessing of a lethal beast, apparently believing that said beast was responsible for the recent killing. The vision ends, Harry is returned to the present, and he tells Ron the dismaying news he just learned - the reason why Hagrid was expelled from Hogwarts.