The Golden Group – Alex Deuyour, Harry Potter, Ron Weasley, Hermione Granger, Kate Nguyen, and Matt Crocitto – break into Bellatrix Lestrange's highly secure vault at Gringotts with help from the goblin Griphook, believing another Horcrux is there. With Griphook's help, they break in, retrieve Hufflepuff's Cup, and escape, though Griphook steals Gryffindor's Sword in the process. Harry has a vision that Lord Voldemort has been informed of the heist and decides to check his Horcruxes. Harry sees that Voldemort's snake familiar Nagini is a Horcrux, and another is hidden at Hogwarts.
Dumbledore's brother, Aberforth, helps the group enter Hogwarts. Voldemort, alerted to Alex and Harry's whereabouts, mounts an assault on Hogwarts. The teachers and students mobilize to defend the school. Ron, Hermione, Kate, and Matt destroy Hufflepuff's Cup with Basilisk fangs from the Chamber of Secrets and Ron and Hermione share an impulsive kiss, much to Kate and Matt's excitement and teasing. Alex and Harry discover Ravenclaw's Diadem is the Horcrux hidden there. The group find it in the Room of Requirement but Draco, Crabbe and Goyle ambush them. Crabbe attacks with Fiendfyre, but is unable to control it; the cursed fire kills him and destroys the diadem. Meanwhile, many allies died during Voldemort's assault, including Remus Lupin, Nymphadora Tonks, and Fred Weasley. All three deaths sadden and upset Sirius Black.
The Elder Wand resists Voldemort's commands. Believing that Severus Snape, the sacked former Headmaster of Hogwarts, having killed the previous master and former Headmaster Albus Dumbledore, is its true master, he has Nagini attack him just as Alex and Harry arrive. Mortally wounded, Snape passes his memories to Alex and Harry. Alex and Harry view them in the Pensieve. The memories reveal that Snape loved Harry's mother and acted as a double agent against Voldemort in an attempt to protect her. He watched over the group, conjuring the doe Patronus, influenced by his love for Lily, who also had a doe Patronus. Dumbledore was dying after mishandling the ring Horcrux and planned his "murder" at Snape's hands. Alex and Harry also learn that Harry is an unintentional Horcrux, unbeknownst to Voldemort, and must die to render Voldemort mortal. Before surrendering themselves, Alex and Harry instruct Neville Longbottom to kill Nagini. Alex shares a final kiss and farewell with a crying Kate, who is mourning Fred, Remus, Tonks, and Colin Creevey's deaths. On the way, they use the Resurrection Stone to reunite with their deceased loved ones, which consists of Lily, Snape, James, Remus, Tonks, and Cedric. Harry drops the Stone as he and Alex meet Voldemort, who casts the Killing Curse on Harry.
Harry awakens in a dreamlike location resembling King's Cross where he is greeted by Dumbledore. Dumbledore explains that Voldemort's original Killing Curse left a soul fragment in Harry, causing their connection. The latest Killing Curse destroyed that soul fragment, allowing Harry to return to life to help Alex defeat Voldemort once and for all or to "go on". Harry returns to life and feigns death. Voldemort spares Alex, attempting to use his life as a bargaining chip for his friends to join him, and calls for a truce at Hogwarts and demands their surrender. Neville, however, pulls Gryffindor's Sword from the Sorting Hat and kills Nagini.
The battle resumes, with Molly Weasley and Sirius killing Bellatrix. Harry and Alex reveal themselves (having hidden away during the chaos) to Voldemort and engage him in a final battle. Having deduced that he and Harry are the true master of the Elder Wand after overpowering Draco at Malfoy Manor. They explain that the Elder Wand's loyalty transfers upon the defeat, not the killing of its former master. Draco, not Snape, had been the Elder Wand's master, having disarmed Dumbledore before Snape killed him. Having disarmed Draco at Malfoy Manor, Alex and Harry now command the Elder Wand. Voldemort casts the Killing Curse at Alex and Harry, but the spell rebounds, killing Voldemort. Before returning the Elder Wand to Dumbledore's tomb, Harry uses it to repair his old wand. He keeps his Invisibility Cloak and leaves the Resurrection Stone lost in the forest. In the aftermath, Alex kisses Kate passionately and they resume their relationship.
Nineteen years later, the trio see their children off to Hogwarts. Harry and Ginny Weasley, Ron's sister, have three children: James Sirius, Albus Severus, and Lily Luna. Alex and Kate have three children: Matthew "Matt" Severus, Severus "Sev" Ronald, Harold "Harry" Sirius Albus, and Kate Lily Deuyour. Ron and Hermione have two children, Rose and Hugo. Matt and his wife have two unnamed children (which will be determined by a comment). Alex and Harry's godson Teddy Lupin, son of Remus Lupin and Nymphadora Tonks, is found kissing Bill and Fleur's daughter Victoire; Draco and his wife are seeing off their son Scorpius. Albus is departing for his first year and worries he will be sorted into Slytherin. Harry reassures him, recounting Snape's bravery, and revealing the Sorting Hat could account for his wishes. Sev is also departing for his first year and worried that he will be sorted into Slytherin.