Alex Deuyour gets his Hogwarts acceptance letter and at Diagon Alley, he learns that Harry Potter, his godbrother and second cousin, is at Gringotts. After getting his school supplies, Alex and his mother Desiree leave to go back to their home.
At King's Cross station, Alex reunites with his fellow godbrother and second cousin Ron Weasley and meets Harry officially. He reintroduces himself to Harry, and explains that the two were best friends as babies. The group also meet Kate Nguyen and Hermione Granger. Alex immediately is smitten with Kate, but finds himself amused by Hermione. Arriving at Hogwarts, Alex warns Harry about his first cousin Draco Malfoy, saying that his parents were Death Eater affiliated. The students assemble in the Great Hall where the Sorting Hat sorts the first-years into four respective houses: Gryffindor, Hufflepuff, Ravenclaw, and Slytherin. Alex is sorted into Gryffindor alongside Harry, Ron, Hermione, and Kate, while Draco is sorted into Slytherin, a house noted for dark wizards.
Alex helps Harry study magic and learn more about his parents and Voldemort, and their innate talents for broomstick flying gets them recruited to the Gryffindor Quidditch team as the youngest Seeker and Chaser, respectively, in a century. While returning to the Gryffindor Common Room, the staircases change paths, leading Alex, Harry, Ron, Hermione, and Kate to the third floor, which is forbidden to students. There they discover a giant three-headed dog named Fluffy. On Halloween, Ron insults Hermione after she shows off in Charms class. Upset, she spends the entire afternoon crying in the girls' bathroom. That evening, a giant marauding troll enters it but Alex, Harry, Ron, and Kate save Hermione; they make up and become close friends after Hermione takes the blame for the incident by claiming she went looking for the troll.
The couplet discover that Fluffy is guarding the philosopher's stone, a magical object that can turn metal into gold and produce an immortality elixir. Harry suspects that Severus Snape, the Potions teacher and head of the Slytherin House, wants the stone to return Voldemort to physical form. Alex doubts this, as Snape is his godfather and honorary uncle, but due to the fact that he is allied with Dumbledore. When Hagrid accidentally reveals that music puts Fluffy asleep, Harry, Ron, Hermione, and Kate decide to find the stone before Snape. Alex reluctantly joins them, to prove that Snape is not the one after the Philosopher's Stone, but rather it is Quirinus Quirrell, the Defense Against the Dark Arts teacher. Fluffy is already asleep, but the couplet face other barriers, including a deadly plant called Devil's Snare, a room filled with aggressive flying keys, and a giant chess game that knocks out Ron.
After overcoming the obstacles, Alex and Harry encounter Quirrell, who wants the stone; Snape had figured it out and had been protecting Harry. Quirrell removes his turban to reveal a weakened Voldemort living on the back of his head. Dumbledore's protective enchantment places the stone in Harry's possession. Voldemort attempts to bargain the stone from Harry in exchange for resurrecting his parents, but Harry sees through his trick and refuses. Quirrell attempts to kill Harry. When Harry touches Quirrell's skin, it burns Quirrell, reducing him to ashes. Voldemort's soul rises from the pile and escapes, knocking out Alex and Harry as it passes through them.
Alex and Harry wake up in the school infirmary. Dumbledore tells them the stone has been destroyed to prevent misuse, and that Ron, Hermione, and Kate are safe. He also reveals how Harry defeated Quirrell: when Lily died to save Harry, a love-based protection against Voldemort was placed on him. At the end-of-school-year feast, Alex, Harry, Ron, Hermione, and Kate are rewarded extra house points for their heroism, allowing them them to lead Slytherin by 110 points; Dumbledore then awards ten points to their housemate Neville Longbottom for having had the courage to stand up to the trio, granting Gryffindor the House Cup. Harry returns to the Dursleys for the summer, happy to finally have a real home at Hogwarts and Alex assures his friends that he will write to them that summer.