On one of their regular visits to Hagrid's house, Harry, Ron, and Hermoine discover that Hagrid has a dragon egg in the fireplace (to let it hatch). Then Hagrid explained that he had won it in a poker game with a mysterious man. But finally, after a lot of arguing, they all decided to give it to Ron's brother in a drop off in the highest tower.
The day of the drop off Harry, Hermoine, and Ron were talking about the plan. Two of them (Hermoine and Harry) would go up to the tallest tower in the invisibility cloak and give the dragon to Ron's brother. But during this Draco overheard and tried to intervene that night. But thanks to the invisibility cloak they slipped by unnoticed while Draco got punished. After the drop off though, they were caught by Filch.
After being caught, Harry and Hermoine given detention with Hagrid. The night after Draco, Hermoine, Harry, and Hagrid go out to the woods to find a missing unicorn. But while they are searching Harry discovers a trail of blood leading to the carcase of a unicorn, whose blood was being drunk by a man in a hood. I chose this scene because it introduces the man in the hood and it causes the kids to start trying to protect the Sorcerer's Stone.