After Harry left the pond. Draco followed him. He stayed far enough to go unnoticed, but close enough not to lose him. He followed him to the library, where he found Granger, then he followed them to Hagrid's hut, to McGonagall's office, and then finally to the third-floor corridor. They entered the door, and closed it behind them. Music played for a little bit, and the silence. He waited for a little while, and when he realized they're not coming back out, he went in.
"What the…" Draco stood there face to face with a three-headed dog. He started singing.
The dog seemed to like his singing, because it fell asleep pretty quickly. He looked around for where Harry and Granger might've gone. He finally found a trapdoor in the floor. He opened it, looked inside. Total darkness. He jumped, and the trapdoor closed. He stopped singing.
He seems to have landed on a soft surface. A mattress he thought at first. It was too dark to tell. But, soon it started to wrap around him. It's definitely not a mattress. After a closer look, he realized it's the Devil's Snare. He remembers it from Herbology. So he just relaxes and slips through.
Where to next? He walks around, until he reaches a room full of what seems to be little winged keys, and both Harry and Hermione on broomsticks flying around.
"Draco! What are you doing here?" Harry noticed him, and was, clearly, surprised to see him.
"I could ask you two the same?" He said. "What is this place?"
"There's no time to explain." Hermione said. "We need to get to the other side."
Draco grabbed a broomstick, and jumped. Soon afterwards, they were making it through the locked door into the next room.
The next room had a huge chessboard. "Checkmate."
Draco took the lead on that one. In no time, they were passing to the next room.
"It smells in here." Draco said.
"That's why…" Hermione pointed to an unconscious troll.
"Snape's already been here." Harry said.
"Snape?" Draco asked. "What's Snape got to do with this? What are we doing here?"
"Snape is helping Voldemort…" at the mention of the name, Draco flinched.
"The Dark Lord" Draco corrected.
"We need to stop them." Harry declared. Draco froze.
"I don't know" Draco seemed hesitant. "I don't think I'm ready to face… him"
"You can go back if you want to, we have to stop them."
"No." Draco objected. "We're first-year students for God's sake!"
"Somebody gotta stop him."
"Somebody! Not you, and definitely not me." Draco said. "Harry, please! He's too dangerous."
"He'll be even more dangerous if we don't stop him." Harry said, turned around, and went through the next door. Hermione looked towards him, and they did the same.
"Ugh, fine." Draco sighs in frustration, and follows suit. As he passes through the door it turns into flame.
"That must be Snape's trap." Harry says, pointing to the potions on the table.
"It's a riddle." Hermione says, picks it up, and reads. "Bad news…"
"Oh no…"
"There is one potion that lets you go through the next door, and one that lets you through the previous door." Hermione explains, while rereading the riddle again.
"And we're three." Draco notes. Hermione nods.
"Well, obviously Harry needs to take the one leading to the next room." Hermione says.
"And you should take the one going back." Harry says to Hermione.
"Why her?" Draco protested.
"Because she would know what to tell Dubmledore when he gets back." Harry reasons.
"Dumbledore's not here?"
"Okay, I figured out the riddle." Hermione says, and hands a potion to Harry. "Good luck!"