What's the matter Potter? Never heard of a wizards duel before?
Harry springs Malfoy's trap
the gang runs into the forbidden corridor
After Harry makes fun of Malfoy for hiding behind his goons, Malfoy challenges him to a duel at midnight in the trophy room.
When leaving for the trophy room Ron and Harry run into Hermoine who tells them to go back to bed and then follows them to the trophy room. When arriving Harry and Ron realize that Malfoy isn't going to show up and that he arrange for Filch, the grounds keeper, to catch them. At that very moment they run into his cat Mrs.Norris and run for their lives.
On the run, the gang makes a wrong turn and winds up in the forbidden sector of the school. In the forbidden zone they find a three headed dog guarding a secret hatch. I picked this scene because it officially stated the main conflict of finding out what was in that hatch.