This is a very simplified summary of the main plot line, so as to avoid any hints about the ending—hope this helps.
In Harry Potter and the Chamber of Secrets, readers see Harry go through his second year at Hogwarts. However, before he gets there, Dobby, the Malfoy's house elf, comes to the Dursley's house to try to stop him from going back to school, and gets Harry into some trouble using magic.
It turns out Dobby was right to try to warn Harry— something is going terribly wrong at Hogwarts. Someone has opened the Chamber of Secrets, which supposedly houses a monster put there by one of the school's founders, Salazar Slytherin (who wanted non pure-blood students to be kept out of Hogwarts). Now students are somehow being paralyzed by the monster, including Hermione. Harry has also been hearing strange voices, and finds the magic diary of Tom Riddle, who was a student at Hogwarts the last time the chamber was opened. Because he's a parselmouth (someone who can talk to snakes), some suspect Harry is the heir of Slytherin, and the one who opened the chamber. However, it's Harry and and his friends who work to find the culprit, and must save someone who has been taken captive by something terribly evil.