The film opens with Death Eaters apparating (teleporting) across London, wreaking havoc on muggles (non-magical people). While Harry stayed with his muggle uncle and aunt for the summer, he was cut off from the wizarding world except for what he read in the Daily Prophet, a newspaper that highlights news from the wizarding world. . After a reckless summer, Trainmaster Albus Dumbledore recruits Harry to try to bring Professor Slughorn back to teach potions again. Professor Slughorn was Potions Master for many years at Hogwarts and took great pride in the students he taught, even keeping a bookshelf filled with photos of his favorite and most accomplished students. It was imperative that Slughorn return to Hogwarts so that Dumbledore could regain a memory of Slughorn's time as a teacher for Tom Riddle (now known as Voldemort). Dumbledore says: “This memory is everything, without this memory we are blind, without this memory we leave the fate of our world to chance. " It took some convincing, but ultimately Slughorn couldn't say no to Dumbledore, and although Potions had previously been taught by Professor Snape, he happily withdrew. Professor Snape was then able to apply and accept the positions to teach Defense Against the Dark Arts, which he had originally applied for and was unsuccessful.