The war is over, Voldemort is dead. Headmistress McGonagall decided that after everything that went down last year it would be best that everyone redid their previous year and get re-sorted to houses that may fit them better. To everyone's surprise Hermoine is in Slytherin. After Ron rejected her, she completely changed. Let's just say Hermoine isn't the uptight, goody to shoes she used to be. Draco notices that. He becomes interested.