The Muggle Prime Minister learns that the dire times that are happening in the UK are not entirely his fault as Prime Minister, but are more to do with the wizarding world. Having been used to dealing with Cornelius Fudge, the Prime Minister is introduced to the new Minister for Magic - Rufus Scrimgeour - who informs him that his staff may not be as Muggle, as he assumed.

Bellatrix Les trange and Narcissa Black pay a visit meanwhile, to Spinner's End, the home of Snape. Bellatrix doesn't trust Snape, but Narcissa is worried about something Voldemort has asked Draco to do. Snape convinces Bellatrix he is trustworthy by saying he will protect Draco and fulfill his task, if he is unable to do so. They seal it with an Unbreakable Vow to her great surprise. Breaking it, will result in death for Snape.

Dumbledore comes to 4 Privet Drive to take Harry to The Burrow, but has two tasks before they go. Firstly, Dumbledore tells Harry he is Sirius' sole heir, and they establish that Kreacher has to serve Harry. Secondly, Harry has to persuade an old-colleague of Dumbledore's, Horace Slughorn, to come out of retirement to teach once again at Hogwarts. At The Burrow, Harry is soon told the news that Bill and Fleur are engaged to be married! Harry is soon caught up on life in the wizarding world, and at Diagon Alley in Madam Malkin's, the trio have a run-in with Draco Malfoy. They later follow him to Borgin and Burkes, where they discover he needs something keeping safe whilst the matching pair of it, is fixed.

The trio get their OWL results and start Hogwarts back on September 1st. Harry and Neville are invited to the 'Slug Club' on the train; a club for those Slughorn thinks has great potential. The school has a surprise when they find out that Slugdon has the Potions job. Snape has finally given the DADA job he has always wanted. It means that Harry can take Potions though at NEWT level, and he is lent an old copy of the textbook until he can get his own copy. Things are not what it seems with that book though. It belongs to the Half-Blood Prince who has left handy notes to make the best potions out there. Slughorn is deeply impressed with Harry's potion's 'skill.' He wins a bottle of Felix Felicus for concocting the best potion in their first class; a potion that brings luck to its drinker.

Dumbledore calls Harry up for private lessons, which are really for them to both explore memories of the life of Voldemort. They learn of how the Gaunt's are the last-living descendents of Slytherin, but are poor. The daughter, Merope, it seems is nothing more than a Squib, who angers her father and brother even more so, when she falls for Tom Riddle; a muggle. Merope and Tom marry, but it seems that she used love potions on him, and when she stopped those, he left her, even though she was pregnant. The session ends with Harry realsiing that Dumbledore has the ring that belonged to the Gaunt family in the memory.

Harry, as new Gryffindor Quidditch captain, holds trials, and establishes the new Gryffindor Quidditch team. After a trip to Hogsmeade, Chaser Katie Bell, is cursed severely from a necklace given to her in the toilets of The Three Broomsticks. Dean Thomas takes over as Chaser for her, and he is also still going out with Ginny, much to the annoyance of Harry. Harry leanrs how Dumbledore told Voldemort he was coming to Hogwarts through his own memory in another of the lessons. Ginny and Ron argue, causing Ron to start seeing Lavender Brown to make up for his lack of kissing experience, much to Hermione's great distress.

Hermione tries to get back at him by asking out Gryffindor McLaggen to Slughorn's Christmas party, but it backfires on her, and Ron and Hermione fall out even further, especially as Ron was annoyed that Hermione had kissed Krum last year. Malfoy gatecrashes the party, and Snape takes him out, to have words with him about his task, making Harry distrust Snape even more. He doesn't have chance to tell Hermione before Harry and Ron leave for The Burrow for Christmas. Percy and Scrimgeour arrive at The Burrow. Percy still hasn't made up with the Weasleys, but Scrimgeour wants to talk to Harry away from Dumbledore, because he wants to know what Dumbledore is up to. Harry refuses to tell him and chastises the Ministry for their previous behaviour towards him over Voldemort's return.

They return to Hogwarts, and the 6th years begin Apparition lessons. Harry tells Dumbledore about Snape and Malfoy's conversation, but Dumbledore sees no reason not to trust Snape. The next memory he learns of was when Voldemort murdered his dad and grandparents in their house. He is shown half a memory of Voldemort at Hogwarts asking Professor Slughorn about Horcruxes, as he falsified the memory he gave to Dumbledore.

Harry shines once again in the next Potions lesson because of the Half-Blood Prince's book, but is unable to get the full memory form Slughorn, and it seems that Skughorn is now avoiding him. On Ron's birthday, Ron accidentally takes some Cauldron Cakes that were meant for Harry that contain a love potion. Harry takes him to Slughorn to be cured, but the mead they have to celebrate turns out to be poisoned, and Ron is rushed to the hospital wing after Harry remembers a Bezoars stone is an antidote to most poisons.

Hagrid comes to visit him in the hospital, and informs them all that Aragog is dying in the Forrest. McLaggen, who lost out on the Keeper's position to Ron, fills in at the next Quidditch match. However, Harry nearly misses the match as he wants to know what Malfoy is up to, and then at the match, is knocked off his broom, so winds up in the hospital wing. Harry asks Kreacher to follow Malfoy, and Dobby joins in with the request as well.

Dumbledore is disappointed that Harry has not found out the missing memory yet from Slughorn, but shares another two with him. In the first, Voldemort learns the whereabouts of his Slytherin's necklace that his mother had, which she had to sell to support herself, and her unborn child. The same woman also has Helga Hufflepuff's cup; she shows up dead two days later ... The second memory is of Voldemort at Hogwarts several years after he had left, asking Dumbledore for a job there; the DADDA job. Dumbledore refuses him, and reveals that since that day, no DADA teacher has ever lasted more than one year ...

Kreacher and Dobby report back to Harry, and he realises that Malfoy keeps disappearing constantly to the Room of Requirement. Harry can't get in at all, but he does bump into Tonks who seems more upset than ever.

Hagrid tells them that Aragog has dies, and Harry decides to take some of the Felix Filicus. He bumps into Slughorn, and convinces him to come to the burial. After getting Slughorn and Hagrid drunk, he manages to get the proper memory out of him, which he takes to Dumbledore. They watch what Slughorn really told the young Voldemort about Horcruxes, and how they are part of the soul hidden in objects. Dumbledore believes that the ring, and the diary of Tom Riddle were two of them. There are four left to find ...

Harry still eager to find out Malfoy's motives, overhears him being comforted by none other than Moaning Myrtle. Malfoy sees him and the two begin to duel. Harry uses one of the spells he has seen in the Half-Blood Prince's book, and Malfoy begins to be slashed to death. Myrtle starts screaming that murder has been done, and Snape rushes in. He fixes Malfoy up, and informs Harry he has just used advanced Dark Magic, and orders him to get his textbooks. He hides the Half-Blood Prince's book in the Room of Requirement. Snape gives Harry detention meaning he misses the last Quidditch match.

Gryffindor win the cup though, and Harry is so overjoyed that he kisses Ginny, who had broken up with Dean. Lavender had also broken up with Ron as well; tired of him avoiding her since his poisoning. Hermione is pleased of course. She looks into the identity of the Half-Blood Prince, and seems to think it was a girl called Eileen Prince.

Dumbledore summons Harry up to his office, and he bumps into Trelawney along the way, who says she has just heard someone whoop in the Room of requirement; Malfoy. Harry gets distracted by this though, when she tells him about her interview 16 years ago at The Hogs Head, and that it was Snape who walked in on her interview with Dumbledore. Harry realises at once that Snape sent his parents to their death. Harry tries to tell Dumbledore that Snape and Malfoy are up to something, but Dumbeldore still trusts Snape. He has summoned Harry to look for one of the Horcruxes. They apparate to the cave, and undertake the obstacles left by Voldemort. Dumbledore has to drink this liquid that seems to make him weaker, but they retrieve Slytherin's necklace. They are attacked by the Inferi, and Dumbledore is very weak, yet, they manage to escape and apparate back to Hogsmeade.

Back there though, they see the Death Mark over Hogwarts, and they rush back to the school. Dumbledore tells Harry to wake Snape, but before he can, Malfoy enters and seizes Dumbledore's wand, just as Dumbledore freezes Harry, who is hidden under the invisibility cloak. Malfoy explains how he let the Deatrh Eaters in through the vanishing filing cabinet that Montague was stuffed in by the Weasley twins. The matching one was in Borgin and Burke's shop in Knockturn Alley, and Malfoy has been fixing it all year. Dumbledore tries to convince Malfoy to come to the good side, but the Death Eaters arrive, and expect Malfoy to kill Dumbledore. He can't do it though, but Snape can, and finishes him off with the Avada Kedavra curse. Harry is released from the freeze spell; Dumbledore is dead.

Snape storms out downstairs with Malfoy, where there are Death Eaters fighting against Order members. Harry goes after them. Snape keeps blocking Harry's curses but stops the others from hurting him; Voldemort wants to kill Harry himself. He reveals himself to be the Half-Blood Prince though before he disapparates outside the grounds. Hagrid and Harry go over to Dumbledore's body, and Harry looks for the necklace. It has gone and a fake one put in its place with a note saying they will destroy the real Horcrux as soon as they can.

No one can believe Dumbledore is gone, but Fawkes is singing a lament. He finds all his friends alive and well though, apart from Bill was seriously mutilated, although still alive, by the werewolf Greyback. Fleur declares she still loves him, and will marry him. Harry tells them the truth about Snape, and they are in shock, as Dumbledore really did trust him. They learn thsat Tonks has been so upset because she is in love with Remus, but he thinks she can do better. They try to find out what Harry and Dumbledore were doing when the school was attacked, but Harry still refuses to tell them.

Hogwarts is to be closed it is decided after Dumbledore's funeral, although many students were taken home early by anxious parents. Hermione was right that Eileen Price had soemthing to do with the Half-Blood Prince's book; she was Snape's mother. Harry breaks up with Ginny at the funeral because he realises he has to go on alone, otherwise Voldemort will probably kill her. Scrimgeour tries to persuade Harry once more for him to tell him about Dumbledore's agenda, but Harry won't.

He announces he will not becoming back to Hogwarts for his 7th year. He's off back to 4 Privet Drive one last time and paying a visit to where it all started in Godric's Hollow. It's then on to find the missing Horcruxes, before battling Voldemort, and if Snape if he meets him along the way ... Ron and Hermione inform him that they will be with him every step of the way.