A/N The rest of the story takes place in their fifth year.
"And time's...UP!" called Professor Slughorn through the mist of brightly colored fog filling the dungeon. Tom tried to shake some of the gobs of salamander intestines off his fingers, but they stayed on, making his hands feel like they were covered in glue. All around the room, people were trying to clean themselves off; the solution that they had to concoct in this lesson, the Draught of Confusion, was among the most messy potions they had made this year.
"Professor?" said Mulciber, straining to make himself heard over the chatter. "Professor, the stone gargoyle isn't working. How are we supposed to wash up?"
Slughorn tried to make his way to Mulciber, but the dense fog made it hard to see. "Oh dear, I suppose we'll just have to let people out to clean up in the bathrooms," he said, after three failed attempts to get there. He gestured to the people nearest to the door. "You by the door, go first. Maybe you can find a teacher to help us fix the gargoyle."
Tom hurried off to the second floor, his mind preoccupied with what he had read in a book that he found in the library...a way to achieve immortality. He had just entered the bathroom and was about to turn on the tap, when he realized that he was in a girls' bathroom. More to distract himself from this unpleasant surprise than anything - he just wanted to clean up and get over with it already - he examined the copper tap. Etched on the side was a small snake.
His pulse quickened as he stared at the snake. He had heard legends of a secret chamber made by Slytherin, hidden inside Hogwarts. Could it be...?
"Open," he hissed.
He stared in a mixture of shock and amazement as the sink shifted and sank right out of sight, revealing a large stone slide that twisted down into the depths of the school. Lighting his wand, he slid down, down, down...
Tom left the bathroom so exhilarated that he didn't care about the potential awkwardness of being seen leaving a girls' lavatory. He had done it! He had achieved his dream of finding a place only he knew of in Hogwarts, a place which was part of his birthright as the heir of Slytherin - the Chamber of Secrets.
This was his weapon in fulfilling his duty of carrying on his ancestor's noble work to rid the school of Mudbloods. He could set the great, bloodthirsty serpent on the vile filth, and it would eradicate them once and for all.
However, he would have to be cautious. He could not actively murder students; that would cause too much suspicion. He must only Petrify students, no matter how badly he was tempted otherwise. He must also keep this a secret, even from Seline. The smallest slip could end up causing his expulsion from Hogwarts, or worse, jailing in Azkaban.
Putting aside his worries for the moment, Tom waved his wand, and words written in rooster blood appeared on the wall beside the lavatory.
"The Chamber of Secrets has been opened. Enemies of the heir, beware."