In the aftermath of battle, Harry goes off in search of a sandwich. He encounters Draco, Who offers a truce. Harry accepts. Draco says let's smoke on it. He leads Harry to Prof Snape's private stash of grade-A grass, hidden in his dungeon office, and they take it outside, away from the castle and the crowd. A saved-by-Fawkes Sev intercepts them and is mad at them for bogeying HIS weed. Stoned hilarity ensues in the forest.
It all began with the weed