In the first book, Harry Potter and the Philosopher's Stone, a baby Harry is sent to live with his aunt (Petunia Dursley) and uncle (Vernon Dursley) after his parents are murdered by Lord Voldemort, an evil wizard. Ten years later, Harry's life with the Dursley's is far from perfect. He is bullied constantly by his relatives, particularly by his cousin Dudley Dursley. After being asked to fetch the post by his uncle Vernon one day, Harry is shocked to find an unusual letter addressed to him. Uncle Vernon refuses to let him read it and rips it up. But more and more letters are delivered to Harry by owls and eventually Uncle Vernon drags his family and Harry away to somewhere where he believes that the letters won't reach them.
During the night, and on Harry's eleventh birthday, they are visited by a giant named Rubeus Hagrid, who hands Harry the letter that Uncle Vernon so desperately tried to prevent Harry from reading. Hagrid is furious to learn that the Dursley's have lied to Harry about his parents' death (they told Harry that his parents died in a car crash), and informs Harry that his parents were in fact a witch and wizard, and that Harry himself is a wizard. From then on, Harry attends Hogwarts school of Witchcraft and Wizardy (the letter that Harry received was from the school, informing Harry that he had been accepted to attend), meets his best friends Ron Weasley and Hermione Granger, plays Quidditch (a sport played on broomsticks), makes an enemy in Draco Malfoy (the school bully), and studies magic.
Harry and his friends get caught up in a whirlwind of adventures when they come across a three-headed dog guarding a trapdoor on the third floor (in the forbidden corridor) of the school. They eventually discover that the dog, who belongs to Hagrid (Hagrid is the keeper of keys and grounds at Hogwarts), is guarding the Philosopher's Stone, a stone that will bring immortality to whoever owns it. Harry, Ron and Hermione try to stop Severus Snape (Harry's potions master, whom he loathes), who they believe is after the stone, from stealing it.
After getting past various obstacles in order to get to the stone, Harry gets a shock as a twist in the plot takes place, and ends up encountering Lord Voldemort, the evil wizard who murdered his parents.
Harry manages to escape, and the book ends with him returning back to the Dursleys for the summer, which leads on to the second book in the series, Harry Potter and the Chamber of Secrets, in which Harry returns to Hogwarts.
