The book starts off with Harry, Ron and Hermione’s children going off to Hogwarts. Albus makes friends with Scorpius Malfoy. Albus Potter is sorted into Slytherin. Rose is sorted into Gryffindor and becomes popular, thus widening the distance between her and Albus.
Draco Malfoy is spread with rumors that Voldemort used a Time-Turner to impregnate his wife Astorio to give birth to Scorpius. He asks for a statement that all Time-Turners have all been destroyed. Amos Diggory also wants Harry to go back in time to the Triwizard tournament and save Cedric.
Harry and Albus argue about Albus’ difficulties at school and Harry says he sometimes wishes Albus wasn’t his son. Harry’s scar begins to hurt for the first time in nineteen years.
Albus offers to help the Diggorys by using a Time-Turner to go back in time to stop Cedric’s death. They steal a Time Turner and sabotage Cedric in the first round of the Tri-wizard tournament.
Harry and Draco discover Albus and Scorpius outside Hogwarts when they are transported back to the present as the Time-Turner has a five-minute limit. Harry tells Albus to keep away from Scorpius as a result.
The next day, there are many changes. Ron is married to Padma Patil and Hermione is a teacher at Hogwarts and Albus is in Gryffindor. They go back in time to sabotage Cedric’s second round. Scorpius can’t find Albus and is informed by Headmistress Umbridge that Harry Potter was killed during a failed coup on the school at the Battle of Hogwarts. Scorpius goes to Severus Snape who is still alive in this reality to ask for help. Scorpius explains that Harry Potter is alive and Voldemort is dead in another world. They use the Time-Turner to go back to the Triwizard tournament and Albus is prevented from casting his spell on Cedric. When they return, Dementors arrive and suck out Hermione and Ron’s souls. Snape is also killed.
Scorpius goes back in time and finds Albus alive. Scorpius wants to destroy the Time-Turner. They hand it to Delphie but realize she is from a family of Death Eaters. Delphie wants Albus to travel in time to make Voldemort ruler by humiliating Cedric or she will kill Scorpius. She takes them back in time, but is struck by Cedric who thinks its part of the tournament. Delphie, Albus and Scorpius pick up the time-turner and Delphie destroys it.
They stop at an unknown period. They see a prophecy, which indicates that Voldemort had a daughter. Albus and Scorpius discover that have been transported to the day that Harry’s parents died. Delphie aims to stop Voldemort from putting part of his soul into Harry and as such when he and Harry fight, the curse will not be rebounded.
Albus and Scorpius send a message to Harry in the present with the date/period they are at. Harry, Ginny, Draco, Ron, and Hermione travel in time to the date. Harry transfigures into Voldemort. Delphie explains to him that she is the child of an affair between Voldemort and Bellatrix Lestrange. The transfiguration begins to fade and Delphie realizes it is Harry. Albus helps his father to beat Delphie. They then hear the real Voldemort arrive to kill Harry’s parents and cannot intervene or risk changing the future. Harry watches his parents being murdered.
In the present, Scorpius asks out Rose Granger-Weasley who accepts. Harry pledges to be a better father and explains that Dumbledore and Snape were great men.