Alex Deuyour and his mother, Desiree, discuss the Order of the Phoenix's plan to move Harry Potter from Grimmauld Place to the Burrow due to his mother's protective spell expiring. During the move, Alex and Harry survive Lord Voldemort's attack but "Mad-Eye" Moody and Hedwig are killed.
During preparations for Bill Weasley and Fleur Delacour's wedding, the new Minister for Magic Rufus Scrimgeour arrives. He informs Alex, Harry, Ron Weasley, Hermione Granger, Kate Nguyen, and Matt Crocitto that Albus Dumbledore left each a bequeath: Ron a Deluminator, Hermione a copy of Tales of Beedle the Bard, Kate a map of Voldemort's possible hiding places for his Horcruxes, Matt the password to Dumbledore's office ("Dumbledore"), Harry the Golden Snitch he caught in his first Quidditch match, and Alex a book about Horcruxes detailing the rituals and how to destroy them (Horcruxes: What They Are and How to Destroy Them). Dumbledore also bequeathed the Sword of Gryffindor to Alex and Harry, but it has gone missing.
News arrives during the wedding that the Ministry has fallen and Scrimgeour is dead. Death Eaters attack and Alex, Harry, Ron, Hermione, Kate, and Matt escape to Number 12, Grimmauld Place. While there, Ron and Matt realize that Harry and Alex's honorary/maternal uncle and godfather, Sirius Black's brother, Regulus, is the R.A.B. who stole Salazar Slytherin's locket from Voldemort. Local thief Mundungus Fletcher later sold it to Dolores Umbridge. Alex, Harry, Ron, Hermione, Kate, and Matt infiltrate the Ministry and recover the locket but they are chased by Death Eaters in a chaotic exit. Alex and Hermione Disapparate them to a forest. Meanwhile, Ron is injured. The group decide to start their journey to discover and destroy all of Voldemort's Horcruxes, starting with the locket.
Attempts to destroy the locket fail. After reading from his Horcrux book, Alex discovers that Basilisk venom can destroy Horcruxes and he and Hermione deduce that Gryffindor's sword can destroy Horcruxes because it is impregnated with Basilisk venom. Ron, affected by the dark locket, is frustrated with their slow progress and irrationally jealous of Harry and Hermione. He argues with Alex and Harry, then Disapparates, leaving Alex, Harry, Hermione, Kate, and Matt to continue their journey. When Harry touches the Snitch to his lips, it reveals a cryptic message: "I open at the close." Hermione notices a strange symbol drawn in Beedle the Bard that is identical to one Luna Lovegood's father Xenophilius wears.
Alex, Harry, Hermione, Kate, and Matt search for the sword in Godric's Hollow and encounter the same strange symbol in a cemetery. Elderly historian Bathilda Bagshot invites them in her cottage, where they find a photo of the young man in Harry's dream who stole a wand from wandmaker Gregorovitch. Bathilda morphs into Voldemort's snake Nagini and attacks Alex and Harry. Hermione disapparates them to safety, but her rebounding spell accidentally destroys Harry's wand. Hermione identifies the man in the photo as dark wizard Gellert Grindelwald. That night, a doe patronus leads Alex and Harry to a frozen pond where Gryffindor's sword lies on the bottom. Alex and Harry dive into the frigid water, but the locket around their necks tighten, strangling them. Ron appears, retrieves the sword, and saves Alex and Harry. They destroy the locket Horcrux with the sword. Ron explains that the Deluminator led him to their location.
A reluctant Alex, Harry, Ron, Hermione, Kate, and Matt visit Xenophilius Lovegood and learn the symbol represents the Deathly Hallows. Many years before, three brothers each received a prize that evades Death: the Resurrection Stone, the Cloak of Invisibility, and the Elder Wand, the most powerful wand known. Possessing all three makes one the Master of Death. Xenophilius secretly summons the Death Eaters, hoping to exchange Alex and Harry for a kidnapped Luna. The six escape, but Snatchers capture them. During a vision, Harry sees an elderly Grindelwald telling Voldemort that the Elder Wand is buried with Dumbledore.
At Malfoy Manor, Bellatrix Lestrange sees a Snatcher has Gryffindor's sword that she believed was in her Gringotts vault. Alex, Harry, Ron, Kate, and Matt are locked into the cellar where they encounter Luna, Ollivander, and Griphook. Upstairs, Bellatrix tortures Hermione. Harry begs for help using a mirror shard in which he believes he glimpsed Dumbledore; Dobby appears in response and helps save everyone while Alex and Harry grab their captured wands from Draco Malfoy. As they disapparate, Bellatrix throws a knife, killing Dobby. Alex and Harry bury him near Bill and Fleur's seaside cottage, an Order safe house. Meanwhile, Voldemort retrieves the Elder Wand from Dumbledore's tomb.