""No," Harry whispered.
"Yes. said Riddle, calmly. "Of course, she didn't know what she was doing at first. It was very amusing... Dear Tom," he recited, watching Harry's horrified face, "I think I'm losing my memory. There are rooster feathers all over my robes and I don't know how they got there... Tom, what am I going to do? I think I'm going mad... I think I'm the one attacking everyone, Tom!""
The first sign of trouble actually invades the sterile, mostly boring and unwelcoming home of the Dursleys - a small creature that calls itself Dobby. Dobby is a house elf - a slave to a wizarding family that seems to be cruel to him and, most importantly, seems to know of a plot to see something awful happen to Harry at school. Mainly through a desire to protect Harry from what is something that is supposedly waiting for him at school, he does a stand-up job of really just ruining a lot of things for Harry. This combines with the fact that Harry has been sad that seemingly everyone he cares for has forgotten to send him mail - which, he discovers, has been caused by Dobby attempting to make him not feel loved by his friends by stealing all of his mail before he can get it.
He warns Harry that he must promise to not return to Hogwarts, and when Harry refuses to promise that, Dobby does a stand-up job of wrecking whatever calm he has at the Dursleys, who, although they are frightened of him, will not abide him making fools of them.
Harry ends up locked away for a day, and he is awakened in the night by a flying car, manned by his best friend Ron and two of his brothers, the twins George and Fred. They break him out of the barred prison that his uncle Vernon has erected for him just in time for Vernon to come crashing through the door and attempt to keep Harry in the room as he fails.
Harry arrives in the Weasley's homestead, where he sees how a poor Wizarding family lives. He discovers that he vastly prefers the lifestyle of the Weasleys to the prissy lifestyle of the Dursleys.
Through a mistake in travelling through Floo Powder, Harry ends up in the wrong place - the black market area for Wizards in London, Knockturn Alley. It is here that he starts to witness the suspicious-as-fuck behavior of the Malfoys - and he nearly gets caught accidentally eavesdropping on the skeevy behavior of Lucius Malfoy, Draco's snobby, obviously villainous father. If he had been caught by Draco, who nearly discovered where Harry had taken to hiding while they spoke, Harry might have gone missing from Hogwarts that year!
From there on, Harry manages to run into Hagrid, who is rather suspiciously wandering through Knockturn. After being reunited with the Weasleys at Diagon Alley, Harry then has a run-in with his future Defense Against the Dark Arts teacher, the greasy (and, possibly, a caricature of Rowling's ex-husband) Gilderoy Lockheart. Lucius make a re-appearance to fight with Arthur Weasley in Flourish & Botts.
Once it's time to go on the Hogwarts Express with all of the Weasley children, Ron and Harry are horrified to discover that the gateway to the train station has been locked to them. Exercizing the sort of logic that one would expect from twelve-year-old boys, the pair take the (illegally) enchanted car for a ride to Hogwarts. Landing the car proved to be a good deal of trouble, though - they land in a very rare and old tree that proves that it can fight back with a fury when it's attacked. They manage to escape and the car become autonomous, escaping into the Forbidden Woods before the tree can manage to destroy it.
Going into the castle, they are immediately discovered and miss out on the sorting ceremony and feast. And, hey, we're not even at the Chamber of Secrets YET!
For a while, things are pretty normal for the three main characters - that is, until Harry comes across a brand of racism that was a reason for Genocide when Lord Voldemort was alive and well - the concept of Pureblood and Mudbloods.
This concept is brought up gently in this book, but as the series progresses, this supposed segregation of pure versus, well, mongrel becomes what is arguably the spinning center of the series. A valid comparison would be the vile war crimes associated with racially motivated - and obscenely cruel - things such as apartheid and racial genocides that have occurred and are still occurring all over the world today. For now, we will just point out that to people like the Malfoys, the Weasleys, although they are "purebloods", technically, are seen as Muggle "sympathizers". and are thus traitors to their race. People like Harry and Hermione, who have Muggle in them, are only fit to be killed off. Draco hurls the insult "Mudblood" at Hermione, and we can immediately see how shocking the insult is in the Wizarding world - Ron tries to damage Malfoy for saying that to Hermione, but instead manages to make himself begin vomiting slugs for a matter of days afterwards.
Harry hears a disturbing, disembodied voice threatening the life of someone or something while he is serving his detention with the self-obsessed Lockheart some time later. Not soon after, something vile happens - someone has petrified the school custodian's beloved cat and has written a disturbing epitaph on the wall next to where the poor thing's been left to hang by her tail. "The Chamber of Secrets has been opened. Enemies of the heir, beware."
It doesn't help in the least that Harry and Ron are found in front of this scene.
This begins Harry's mistreatment by some of the student body (*cough, cough* frightened, pussy Hufflepuffs *cough, cough*) who think that he must be the supposed Heir, especially following the revelation that Harry is a Parseltongue - something associated only with evil wizards.
This only gets worse when it is discovered that Colin Creevey, a fan of Harry's, is discovered petrified in the same manner as Mrs. Norris (the cat), Harry is nearly killed by an enchanted ball in a game of Quidditch that tries to drop him off of his flying broom throughout the game and the three main characters are caught up with trying to prove that Malfoy is this feared Heir. Their detective work ends up proving that as oily as Draco is, he only wishes that he could be the Heir - and it ends with Hermoine in the nurse's wing, partially transformed into a furry (no seriously).
Things get hairier, as more people end up petrified - but things take a desperate turn when Ron's little sister, Ginny, is taken into the supposedly mythical Chamber of Secrets, and the school is set to be closed. A magical diary from a boy names Tom M. Riddle is discovered by harry, and he learns that he can communicate with the school prefect from fifty years ago by writing in it. Tom reveals to him that fifty years ago he turned Hagrid in as the person who was involved in the Chamber of Secrets incident - Hagrid's obsession with a large spider. This lead to Hagrid being dismissed from school, and due to Dumbledore's kindness, he is kept on, instead, as the school's Gamekeeper.
When they decide to confront Hagrid, they arrive in time to witness him being shipped off to the fearsome prison of Azkaban. Before he leaves, however, he tells the boys to "follow the spiders". Dumbledore similarly seems to sense them, and he tells them that all those who seek help will always find it in Hogwarts. Some time later, Hermione ends back up in the nurse's for the second time - petrified.
Some time later, they do manage to find a good deal of spiders, which they follow into the Forbidden Forest, only to discover both a brood of terrifying, large spiders - and a dead end that nevertheless proves that Hagrid had nothing to do with the incident fifty years prior.
To Hagrid, giant, scary-ass spider = fluffy best friend
Things get worse when Dumbledore is unanimously taken out of power in the school - thanks to Lucius Malfoy. Now Harry feels compelled to save the school and all of the people endangered by what has happened.
The chamber, it is revealed, in in an abandoned girls' bathroom. Harry is able to get in, because he speaks Parseltongue, and once in, he is separated from Ron when Lockheart, whom they have forced to come with them, attempts to attack them and it backfires, due to the fact that he tries to perform a spell with Ron's damaged wand, causing the spell to backfire on him. Going further, he find Ginny near-death, and is surprised to discover that the Riddle boy is now physically there - and in control of the large basilisk that has been trying to kill people in school, but succeeded mainly only in petrifying people in the castle.
"Tom Marvolo Riddle" - When mixed up, the letters in his name can also tead, "I am Lord Voldemort." Some kids are too clever for their own good...
Tom, it is revealed, is very much like Harry, but they share one other similarity that is shocking - Tom Marvolo Riddle is the boy who would later become Lord Voldemort, and he had left a piece of himself in his diary to wreck havok later in life. He was the heir of Slytherin and had used Ginny, who had gotten ahold of his diary, to do his dirty work.
He sics the monstrous snake on Harry, who would have surely died, especially following the snake breaking off a poisoned fang into Harry, if not for the phoenix Fawkes who is the faithful companion of Dumbledore who appears int he chamber, blinding the monster snake, providing Harry the means to summon the sword of Godric Griffindor and then saving Harry from the poison of the fang.
Destroying the Basilisk, Harry then uses the fang that almost killed him to destroy the charm of the diary.
All that is left is to escape the chamber with the three people who entered the chamber and Harry arrives in Dumbledore's office to see the Headmaster's returned - along with the Weasleys, grieving over the fate of Ginny, right before they enter with Ginny, well alive, in tow. it is then revealed that Lucius engineered everything that happened, more or less directly, slipping Voldemort's old diary into Ginny's cauldron while in the bookstore. Harry reveals this, and Dumbledore lets it be known that Lucius no longer has any place on the school's board.
Harry then recognizes that the house elf trailing Lucius is none other than Dobby. Harry tricks Lucius into throwing clothing at Dobby - the presentation of clothing to a house elf in the way to break the binding of a house elf to that person's family.
Lucius, livid at the loss of his family slave, shows his true cards when he attempts to attack Harry - only to be blocked and retaliated against by the newly-freed Dobby. Although it might have been best to report Lucius attacking him, it nevertheless in a good moment.
I know, it must be embarrassing enough to get your ass beat by this guy, but Harry should have STILL reported that asshole attacking him to SOMEONE. The guy's a violent control freak - and he was the ENTIRE reason that Ginny spent the school year as a Voldemort zombie - get some sense, kid!
The book ends with things back to as normal as they would ever be - Hagrid is let out of Azkaban to return to Hogwarts and Draco is no longer a big as a shit as he was before, now that his father has been publicly disgraced.
This sort of ending is a thing that is unheard of, as the series went past the fourth book later on, so it's best to enjoy this sort of win for Harry while it's here.