The novel begins with the wizarding world's celebration of the downfall of Lord Voldemort, an evil, powerful and cruel Dark wizard. After killing Lily and James Potter, Voldemort had attempted to kill one-year old Harry Potter, but the curse rebounded upon him, destroying his body and leaving a lightning-bolt scar on Harry's forehead. Professors Dumbledore and McGonagall and Gamekeeper Rubeus Hagrid leave Harry on the doorstep of his ultra-conventional and unsympathetic Muggle relatives, the Dursley family. Petunia Dursley is the sister of Harry's mother Lily, although Petunia never approved of her magical sister. The Dursleys resolve to conceal Harry's magical heritage from him, and maltreat him while overindulging their own son Dudley. They tell Harry that his parents' deaths and his scar were caused by a car accident.<p />
Shortly before Harry's 11th birthday, he receives a mysterious letter addressed specifically to him. His outraged uncle reads and burns the letter before Harry can see it. The sender does not capitulate, and the Dursleys receive increasingly greater numbers of the same correspondence over the next few days. Soon, his uncle becomes so paranoid that the Dursleys, with Harry in tow, hide in a hut on a small island to escape. That night, Hagrid bursts through the hut's locked door. As Hagrid holds the Dursleys at bay, Harry finally reads his letter in which he learns he has been invited to study magic at Hogwarts School of Witchcraft and Wizardry. The next day, Harry and Hagrid leave the hut and head to London's magic hub Diagon Alley via famous wizarding pub The Leaky Cauldron. Harry enters the wizarding world for the first time, learns to his surprise that he is famous for his defeat of Voldemort, and meets the new Hogwarts Defence Against the Dark Arts teacher, Professor Quirrell. He returns to the Dursleys and a month later, takes the train to Hogwarts from Platform Nine and Three Quarters at King's Cross Station. On the trip to Hogwarts, Harry befriends Ron Weasley and meets the forgetful Neville Longbottom and Hermione Granger, a precocious Muggle-born witch.<p />
Upon arrival, the Sorting Hat places Harry, Ron, Hermione and Neville in Gryffindor House, one of the four houses at Hogwarts. Draco Malfoy, an arrogant and elitist student, gets placed in Slytherin. At the end of his first week at Hogwarts, Harry and Ron discover that the wizarding bank Gringotts was robbed, and a vault that Harry and Hagrid visited had been the subject of the burglary. Later, Harry discovers he has a talent for riding broomsticks, and after a broom-mounted game of keep away with Malfoy over Neville's Remembrall, he is recruited to join Gryffindor's Quidditch team as Seeker. He is the youngest Quidditch player at the school in a century, much to Malfoy's displeasure.<p />
Harry, Ron, Hermione and Neville, in Hogwarts' halls late at night, accidentally stumble across the door to a corridor on the third floor that they had been warned not to enter, finding in the corridor a monstrous three-headed dog, christened Fluffy by Hagrid, that guards a trapdoor. On Halloween, Quirrell informs everyone that a troll has entered the castle and is in the dungeon. As the rest of the students are ushered to their dorms, Ron and Harry remember overhearing that Hermione is in the girls' bathroom crying because Ron insulted her, and they realise that she wouldn't know about the troll. The two of them go to the girls' bathroom, fight the troll, and save Hermione. When confronted by Professor McGonagall over why they did not go back to their dorms when directed, Hermione defends the boys and takes the blame, which results in the three becoming best friends.<p />
At Harry's first Quidditch match, Harry's broom becomes possessed, nearly knocking him off. Hermione sees Professor Severus Snape, the sinister Potions master, staring at Harry and mouthing words, causing her to believe that Snape has cursed the broom. Hoping to save Harry, she runs and accidentally knocks over Professor Quirell. Hermione sets Snape's robes on fire, distracting him and others and allowing Harry to survive and catch the Snitch to win the game for Gryffindor.<p />
At Christmas, Harry receives an Invisibility Cloak, once belonging to his father, which renders its wearer invisible. Harry uses it to explore the Restricted Section in the library to learn about Nicolas Flamel, a name Hagrid let slip when confronted about his knowledge of Fluffy. On being discovered in the library by caretaker Argus Filch, Harry escapes to a disused classroom in which he finds the Mirror of Erised, which shows Harry his family. After three nights of returning to the mirror, once accompanied by Ron, Harry is confronted by Dumbledore, who explains that the mirror shows our deepest desires: Harry sees his family, while Ron sees himself achieving more than his older brothers. Dumbledore tells Harry the mirror is to be moved, and if he sees it again he will be prepared. Eventually, Harry learns (through Dumbledore's Chocolate Frog card) that "Nicolas Flamel is the only known maker of the Philosopher's Stone, which produces the Elixir of Life which will make the drinker immortal."<p />
Harry sees Snape trying to get information from Quirrell about getting past Fluffy; Quirrell says he does not know what he's talking about. Harry, Ron, and Hermione are sure that Snape is trying to steal the Philosopher's Stone in order to restore Lord Voldemort to power, but Hagrid denies it. While at Hagrid's hut, the trio discover a dragon egg Hagrid is nursing in a fire. The egg hatches a Norwegian Ridgeback dragon, Norbert. The friends are nervous for Hagrid, since dragon breeding has long been outlawed in the wizarding world, and Hagrid, with his reckless nature, has long nursed a strong desire for a dragon. Finally, Harry, Ron, and Hermione are able to convince Hagrid to let Norbert go live with other dragons of his kind with Ron's older dragon trainer brother, Charlie, in Romania. They arrange for the dragon, (now quite large in size), to be picked up by friends of Charlie's.<p />
Harry, Hermione, and Neville are caught out late at night while sending off Norbert (Ron is in the hospital wing, being treated for a bite from Norbert), and they are forced to serve detention with Hagrid in the Forbidden Forest along with Draco, who had been spying on them. Harry sees a hooded figure drink the blood of an injured unicorn, which causes pain in Harry's forehead scar. Firenze, a centaur, tells Harry that it is a monstrous thing to slay a unicorn, let alone drink its blood, that unicorn blood sustains life but gives the drinker a cursed life in return, and that the hooded figure is in fact Voldemort.