The book opens with Snape and Yaxley, a fellow Death Eater arriving at Malfoy Manor (their new headquarters) to inform Lord Voldemort of Harry Potter's move from 4 Privet Drive into a safe house, as Harry's 17th birthday approaches. Charity Burbage, Hogwart's Muggle Studies teacher is held suspended above the table and is killed. Bellatrix is infuriated to learn her niece, Nymphadora Tonks has married Remus Lupin.

Meanwhile at 4 Privet Drive, Harry prepares to leave. He has to convince the Dursley's to take Order protection, and as usual Uncle Vernon needs some convincing, believing Harry to be after the house because of rocketing house prices! Harry reads a tribute in The Daily Prophet to Dumbledore from his Hogwarts friend, Elphias Doge, and discovers that Dumbledore's family had faced scandal. In the same edition, Rita Skeeter is advertising her 'truthful' biography of Dumbledore's life. Dedalus and Heista show up at Privet Drive to take the Dursley's to safety, but before they leave, Dudley thanks Harry for saving his life the previous summer and the two shake hands. They leave and Harry is left alone until the Order, Hermione, Mundungus and the Weasleys arrive.

They use the Polyjuice Potion and create six other Harry Potters to confuse the Death Eaters if they are waitng for them. The real Harry is with Hagrid, and Voldemort soon learns he is the real one when Harry spares Stan Shunpike's life. After a violent mid-air battle, Hagrid and Harry manage to crash from Sirius' flying motorbike into the protected safehouse of Tonks' parents, where they take the portkey to The Burrow. Not everyone makes it back though. Hedwig was killed, as was Mad-Eye Moody, and George lost an ear in the battle.

Mrs Weasley tries to find out the trio's plan, but there is the upcoming wedding of Fleur and Bill to see to. Hermione has already wiped her parent's memory and sent them to Australia and Ron has turned the ghoul into the attic into him, but with spattergroit to try and disconnect the Weasleys from Harry. Harry celebrates his 17th birthday, but before the proceedings can begin, Scrimgeour comes to The Burrow to tell Harry, Ron and Hermione what Dumbledore left them in his will. Ron has been left Dumbledore's deluminator, Hermione a book called 'The Tales of Beedle the Bard, and Harry his first golden snitch and Gryffindor's sword. Scrimgeour however does not give him the sword saying it is Hogwart's treasure. The next day is the wedding, but a message comes during the reception that Scrimgeour has been killed and the Ministry taken over. The protection of The Burrow has gone and Death Eaters start to appear.

Harry, Ron and Hermione make it to Tottenam Court Road, but are soon found by two Death Eaters, who they deal with. They decide to take refuge in Grimmauld Place as they know Mad-Eye Moody has put protections on it to stop Snape from revealing it to Voldemort. Harry explores Sirius' room and discovers someone has been there looking for something. Passing down the corridor, he notices the bedroom as Sirius' brother and realises that Regulus Arcturus Black is R.A.B who removed the locket. They do not find it, and search Kreacher's belongings when they realise that they had seen the locket in the Black household. Harry summons Kreacher from Hogwarts and they learn Voldemort used Kreacher to put the locket in the stone basin on the island. Regulus afterwards decided to remove it and died trying, but told Kreacher to destroy it. Harry gives Kreacher the fake locket and Kreacher is so happy to have Regulus' locket that he recognises Harry as his Master properly, and goes to find 'dung on Harry's order to discover who 'dung sold the locket to, as 'dung stole the locket from Kreacher. When Kreacher fetches him back, they realise that it is Umbridge that the locket went to.

The trio decide to stalk the Ministry of Magic to get in to get the locket back. Meanwhile Harry is sharing Voldemort's experiences once again and learns that Voldemort is after information on wands. Finally, the trio execute their plan and enter the Voldemort-controlled Ministry. The Ministry are conducting trials against Muggle-borns accusing them of stealing magic, and after getting the locket from Umbridge, the three try and save as many Muggle-borns as possible. However, whilst trying to flee the Ministry, Yaxley manages to grab Hermione and thus, learns of Grimmauld Place. She manages to shake him off and they apparate to a forest where they cast protective spells around their tent. After hearing voices, they discover that the sword that was in Snape's office of Gryffindor is a fake by goblins who are laughing about the matter. The fake sword has been sent to the LeStrange's vault after an attempt to steal it, but they do not know where the real sword is. Tensions build up between the three, and the evilness of the Horcrux's presence causes Ron to leave Harry and Hermione. Hermione is devastated.

Hermione and Harry go to Godric's Hollow where they discover Harry's parents' graves and those of Dumbledore's mother and sister. There is also the house that the Potters lived in left there as it was when Voldemort killed them. They have gone to the village to see Bathilda Bagshot; friend of the Dumbledore's to see if she can help with the sword. However, it is a trap. Voldemort has possessed Bathilda with Nagini, his snake who calls for Voldemort. Harry and Hermione manage to escape. In the next issue of The Daily Prophet, Rita Skeeter continues with her attempt to tell the world about the true Dumbledore and points out his best friendship with Grindelwald, the wizard Dumbledore famously won at a duel in 1945 after Grindelwald killed many. She hints that Dumbledore was not the Muggle-lover he seemed, and again alludes to the family's supposed dodgy past. Harry and Hermione move forest.

In this forest, Harry wakes in the night and sees a silver doe. He follows it and it is there he sees the sword of Gryffindor in a lake. He dives in and the Horcrux tries to kill him, but Ron appears and saves him. They smash the locket with the sword. Ron explains what happened to him, and tells them that anyone who says 'Voldemort' summons people to investigate and breaks any spells. Hermione decides they need to see Luna Lovegood's father because of a connection she'd like to pursue. In the book that Dumbledore left her there was a symbol that she also seen in Godric Hollow's graveyard. The same symbol was worn by Luna's father at the wedding. At the Lovegood's house, Xenophilius tells them the fable of the Deathly Hallows - a wand, an invisibility cloak and a resurrection stone that enable the holder to master Death. Harry already has the invisibility cloak and realises he is descended from the Peverell's who were the three brothers that each had one of the hallows. Xeno tries to sell the three out to Death Eaters because they have kidnapped Luna, but the three escape. Harry is determined to go after the other two Hallows, but Hermione reminds him that it is Horcruxes that they are after.

Harry forgets about the Voldemort taboo and says it. The three are set upon and are taken to Malfoy Manor. Bellatrix keeps Hermione behind to torture and the others are thrown in the dungeon. Harry sees a flash of blue in the mirror Sirius gave him and yells for help. Dobby appears and takes Luna, Ollivander, Dean Thomas and Griphook, the goblin to safety who are all there. The Death Eater's hear the noise and send Wormtail to investigate. Only Ron and Harry are left. Wormtail hesitates in revealing the others escape and his betrayal to Voldemort to help Harry causes his silver hand to strangle himself to death. Harry and Ron go upstairs and manage to rescue Hermione when Dobby returns and smashes a chandelier over them. They disapparate with Dobby but Dobby has been stabbed by Bellatrix's knife and arrives at Shell Cottage dying. His last words are Harry Potter. Harry buries him in the garden manually, not magically.

Harry asks Griphook, the goblin to help them break into Gringotts because Harry realises the cup of Hufflepuff is there with Bellatrix's reaction to the sword of Gryffindor. Lupin arrives at Shell Cottage to tell them that Tonks has had a boy called Teddy, and asks Harry to be godfather. The three leave the cottage with Griphook after Hermione transforms to Bellatrix and they head to Gringotts. They manage to get the cup from the vault, and escape on a dragon, but Griphook takes the sword with him. They cannot yet destroy the Horcrux, and more disaster looms as Voldemort realises Dumbledore has the Elder wand and destroys his marble tomb to get it off him.

Voldemort realises that Harry must know about the Horcruxes at this news and goes to check the others are safe. Harry reads his mind and realises there is one at Hogwarts. They head there and are rescued by Aberforth who is the barman of The Hog's Head. It is Aberforth who sent Dobby to help because he had the magic mirror. He tells them the truth about the Dumbledore family. Their younger sister Ariana was sick and couldn't control her, accidentally killing their mother. Albus, Aberforth and Grindelwald were arguing and Ariana lost control. One of them accidentally killed her, and Aberforth has blamed Dumbledore since. Harry tells him that Dumbledore was tortured by that for the rest of his life - it was what he saw when he drank from the basin on the island. They ask Aberforth for help into Hogwarts and a picture of Ariana reveals a passage to Hogwarts where Neville takes appear and takes them into the castle.

Everyone is thrilled to see Harry thinking he has come to free the school, but he must get the Horcrux. People flock in to help, including Percy. Harry however realises that it is the lost diadem of Ravenclaw he needs to destroy after going to the Ravenclaw common room. McGonagall appears after Harry took down the two Death Eaters at the school second to Hogwart's headmaster, Professor Snape. He tells her that Voldemort is on his way and they must stop him from entering Hogwarts for as long as possible. She rouses the staff and sets to evacuating the pupils. Snape flees after the Head of Houses try and overthrow him. Ron and Hermione find Harry - they'd gone down to the Chamber of Secrets to destroy the cup, and Harry tells them he knows where the Horcrux is of Ravenclaw. It is in the Room of Requirement. Malfoy, Crabbe and Goyle are waiting there for them.

The diadem is destroyed and the trio rescue Malfoy and co. after Crabbe uses a cursed fire on the room. He is killed by the fire. The battle between good and evil meanwhile is raging on inside Hogwarts, and Fred is killed. Harry has to go after Voldemort now and sees he is in the Shrieking Shack. Harry goes with his invisibilty cloak and witnesses Voldemort killing Snape. Voldemort leaves and Harry goes to Snape. He gives Harry his memories, takes a look into his mother's green eyes, and dies. Harry enters the memories and learns that Snape knew Lily before they were at Hogwarts together. They lived near each other, and Snape even knew Aunt Petunia! At Hogwarts though, even though they were best friends and in different houses, the two parted ways when Snape became too keen on the Dark Arts and accidentally called Lily a 'Mudblood'. Snape still loved Lily however. He asked Voldemort to spare Lily when the prophecy was revealed, and when Voldemort wouldn't, he asked Dumbledore for help. Snape swore to protect Harry forever as that was what Lily would have wanted. Snape reveals that Dumbledore knew he was going to die and only had a year left when the ring cursed his hand, and it was agreed from the start that Snape would kill Dumbledore to spare his suffering, but also at the right moment, and to stop Draco's soul from being destroyed. He also learns that Snape gave him back Gryffindor's sword. The doe was his patronus, and that Harry according to Dumbledore has to be killed by Voldemort. Harry leaves the memory knowing that he must die, but also that Snape had always been on his side. Harry had been wrong.

Harry goes to Voldemort to face his death, and tells Neville on the way the importance of killing Nagini. Harry smashes open the snitch to get the resurrection stone out of it, and his parents, Sirius and Lupin (who died in the battle, along with Tonks) walk him to Voldemort. He does not fight Voldemort back, but stands there to die...

Harry awakens at Kings Cross where Dumbledore is waiting for him. He tells Harry how proud of him he is, and then reveals what he guessed to be the true nature of the prophecy. Harry was one of Voldemort's Horcruxes, and that was what Voldemort had just destroyed. The "neither live whilst the other survives" reference means that Harry is still alive because Voldemort regenerated out of Harry's blood. As Voldemort is still alive, so is Harry, but Harry now has his own soul intact. He gets a train out of Kings Cross and wakes up in the forest. Voldemort sends Narcissa Malfoy over to see if Harry is dead. She asks if Draco is alive, and Harry says yes. Narcissa announces Harry is dead. Voldemort gets Hagrid who is tied to a tree to carry Harry's 'dead' body to show the fighters he has fallen. The DA leave the great hall and are shocked to see Harry dead at Voldemort's feet. Neville manages to kill Nagini though as Grawp arrives with more fighters to take on Voldemort. Harry slips under his cloak and disappears. The fight starts once more and Mrs Weasley kills Bellatrix. Harry reveals himself and the final battle begins between Harry and Voldemort. They circle each other and Harry reveals that he is the true master of the Elder wand and that he is stronger than Voldemort. He also tells the truth about Snape. The two cast spells and Harry is right. The Avada Kedavra spell backfires on Voldemort, and he is killed. Everyone cheers! Harry returns to Dumbledore's office and decides to leave the resurrection stone in the forest where it fell and give the Elder wand back. He will however, keep his cloak.

Nineteen years later ... Harry and Ginny are at Kings Cross to take second son Albus Severus to start Hogwarts. Eldest son James is already a student there, and daughter Lily has two years until she starts. They meet Hermione and Ron whose eldest child Rose is also about to start at Hogwarts. Their younger son Hugo is also too young to attend. They pass Draco and his son Scorpius who is also starting school, and Lupin's son Teddy is already on the train snogging a girl! Albus is terrified that he will be put into Slytherin and Harry confides that he has the choice to choose, because Harry chose Gryffindor over Slytherin. The children get on the train and leave for school with them waving the children off. Harry checks his scar, but there is nothing after nineteen years ... "All is well."