J. K. Rowling’s Half-Blood Prince Visits to discuss and the recent tragedies he’s caused for Muggles and wizards. Fudge claims he’s been fired for being unable to control the Dark wizard and introduces his successor, the brusque.
Narcissa Malfoy visits England clandestinely with her sister. Snape offers to help Narcissa’s son with a mysterious Dark Lord job. Snape tells Bellatrix he’s served Voldemort for years and has been a spy at Hogwarts.
Meanwhile, is at the Dursleys’ mourning his godfather’s death. He takes Harry with him to convince a retired colleague to teach at Hogwarts. Dumbledore advises Slughorn not to get “collected” by Harry’s mystery. Dumbledore informs Harry that the Weasleys have special Ministry security for his protection before dropping him off at the Burrow. Harry spends the summer at the Burrow with and, but they are disturbed by Order of the Phoenix members.
While collecting school supplies in Diagon Alley, Harry, Ron, and Hermione meet Draco Malfoy and follow him to Borgin & Burkes, where he threatens the owner if he can’t repair a curious artefact. Harry believes Malfoy is a Death Eater, but no one else does. Malfoy discovers Harry eavesdropping on him gloating about serving Voldemort, fractures his nose, and confines him in a train compartment, making him late to the school’s inaugural feast.
Dumbledore announces to Harry’s amazement and terror that Slughorn will teach Potions and Snape will teach DA. All sixth-years are stressed by this year’s workload, but Harry is helped in Potions by a curious secondhand textbook he finds in the classroom. Despite Hermione’s objections, Harry learns from the Prince’s notes. This fascinates Slughorn, who invites Harry and Hermione to office gatherings while ignoring Ron. Harry also becomes Quidditch captain. Hermione Confunds the blustering and pompous when he tries out for Ron’s Keeper job.
Dumbledore uses the Penseive to show Harry Voldemort’s family history and youth during Harry’s lessons. Harry learns that Voldemort is from the once-wealthy but now-poor Gaunt family. In the Penseive, he sees Voldemort’s grandfather and uncle harass his mother; Dumbledore tells that they’re currently in Azkaban. Merope uses her magic to cast a love spell over a Muggle she loves. After they marry and she falls pregnant, she lifts the charm, hoping he’s developed actual feelings for her. He abandons her, and she dies of grief shortly after giving birth to their baby. Harry sees Dumbledore retrieve Tom Riddle from his orphanage, where the matron shows his odd powers and sadistic habits. Harry sees a partially-obscured memory of Professor Slughorn where Riddle asks about Horcruxes. Dumbledore says that Slughorn modified with the memory to hide an act he’s ashamed of. He tells Harry to extract the genuine memories from him.
Meanwhile, frightening circumstances threaten Hogwarts students. Unknown individual puts a Gryffindor Quidditch player under the Imperius Curse and charges him with delivering a weird package to Hogwarts. She almost dies after touching the cursed jewellery and spending months at St. Mungo’s Hospital. Ron unintentionally drinks a love potion meant for Harry. Ron is saved by Harry’s prompt action when Professor Slughorn gives him a glass of poisoned mead after formulating an antidote. Harry is confident Draco and Snape are behind these unsuccessful plots, but Ron and Hermione are dubious. Dumbledore and McGonagall admonish Harry for making allegations without evidence.
The three pals also face romantic issues. Ron and Hermione are taking a fresh interest in each other, and she wants to ask him to Slughorn’s Christmas party. Ron is angered by Slughorn’s lack of care for him and fights with Hermione. Ron starts dating, whose public displays of affection upset Harry and hurt Hermione. In a corridor, Harry and Ron see Dean Thomas kissing. Ron accuses his sister of inappropriate behaviour, and she reacts, telling him not to regulate her love life. After seeing Ron’s reaction to Dean, Harry feels scared to act on his love for Ginny.
Harry neglects to safeguard Slughorn’s memories and is punished by Dumbledore. He drinks Felix Felicis, a fortunate potion he won in a contest. Harry visits the acromantula’s burial at’s cabin, taking Slughorn along. Harry uses Slughorn’s emotional mood to encourage Hagrid to share the recollection. Harry hurries to Dumbledore’s office after obtaining the memory to watch Slughorn teach Riddle about, a type of Dark magic that allows a wizard to split his soul and achieve immortality. Dumbledore believes Voldemort has buried several Horcruxes; destroying them will ensure he can never rise again.
Harry stalks Draco despite Dumbledore’s reassurances. One day, he finds his opponent sobbing in a restroom and telling a ghost that Voldemort will kill his family if he doesn’t do anything. Harry performs a strange spell from his Potions textbook to slash Draco’s body, almost killing him. Harry must miss the Gryffindor Quidditch final because Snape gives him Saturday detention. Everyone is unhappy with him, but the team still wins; in celebration, Harry kisses Ginny in the common room and they start dating with Ron’s blessing.
Harry enjoys his new connection with Ginny for several weeks before Dumbledore calls him to destroy one of Voldemort’s Horcruxes. Dumbledore discovers a hidden pool in a secluded seashore cave. They row to a central island and locate a Horcrux in a liquid basin. Dumbledore is forced to take the potion, which weakens him and induces horrific hallucinations. Harry must lead their retreat across a lake filled with magical dead bodies (Inferi). Dumbledore leaves the cave and apparates to Hogsmeade, where he warns that the Dark Mark has appeared over Hogwarts, meaning Death Eaters are within.
Harry and Dumbledore borrow brooms from Rosmerta and fly to Astronomy Tower. Dumbledore immobilises Harry under his Invisibility Cloak, leaving himself defenceless. Soon, Draco confesses his intention to kill Dumbledore. He says Voldemort gave him this mission and he’s behind the cursed jewellery and poisoned mead. Dumbledore begs him to swap sides, promising to safeguard him and his family. Snape, with with several other Death Eaters, appears and kills Dumbledore.
Harry, freed by Dumbledore’s death, chases Snape downstairs. The professor taunts him and confesses he’s the Half-Blood Prince when he curses him. Disapparating, he and Draco escape. Harry hurries to the hospital wing where the Order of the Phoenix is grieving Dumbledore’s loss and caring for, who was ravaged by a werewolf. Seeing her daughter-in-devotion law’s to her fiancé, reconciles with her.
Hogwarts’ future is uncertain after Dumbledore’s death and the new security breach, yet all the pupils attend his burial. The celebration is attended by Dumbledore’s friends, merpeople and centaurs. Harry is numb at ceremonies. No adult can protect him from Voldemort, and relying on authoritative figures was a mistake. After the burial, he breaks up with Ginny because he doesn’t want Death Eaters to use her against him. She doesn’t want his protection, but accepts it.
Even if Hogwarts reopens, Harry will depart to find the Horcruxes, he informs Ron and Hermione. His companions quickly swear to join him on this quest. Ron tells him they must celebrate Bill and Fleur’s wedding before leaving. Harry is encouraged that marriages can still happen in this new period of volatility. Even though the journey ahead is hazardous, he’s supported by his two best pals.