The three main characters are Ron, Hermione and Harry. Harry thinks Draco is a Death Eater. In a clothing shop, Draco would not let the shopkeeper roll up his sleeve, so Harry thinks he has been branded with the dark mark.
Dumbledoor brought Harry to a retired teacher to convince him to come back. Later, Harry didn't buy his potions book, so he is loaned an old copy that used to be owned by the Half blood prince. It is filled with short cuts and other helpful things.
Harry is convinced that Draco Malfoy is up to something in the room of requirement, but Harry can't get in. is Draco a death eater? find out in Harry potter and the half blood prince.