At Hogwarts, Harry begins learning wizardry and also discovers more about his past and his parents. He gets recruited for Gryffindor's Quidditch (a sport in the wizarding world where people fly on broomsticks) team as a Seeker, as his father was before him. One night, he, Ron and Hermione discover a large three-headed dog on a restricted floor in the school. The dog is guarding the Philosopher's Stone, an item that can be used to grant its owner immortality. Harry concludes that his potions teacher, the unfriendly Severus Snape, is trying to obtain the stone in order to return Voldemort (who Harry encounters in the Forbidden Forest where he, Ron, Hermione and Draco Malfoy are serving detention by helping Hagrid look for an injured unicorn after being caught wandering around at night) to a human form.