Harry Potter and the Half-Blood Prince recap

With war now open, Harry's sixth year mixes ordinary school life with Dumbledore's quiet campaign against Voldemort. Dumbledore gives Harry private lessons, using collected memories to trace Voldemort's history from his orphan boyhood as Tom Riddle. Harry excels at Potions thanks to an old textbook annotated by a mysterious 'Half-Blood Prince,' and grows sure that Draco Malfoy has become a Death Eater plotting something inside the castle. Using a lucky potion, Harry retrieves a hidden memory revealing Voldemort's secret to immortality: Horcruxes, objects into which he split his soul, several of which must be destroyed to make him mortal. Dumbledore and Harry go to a hidden cave to destroy one, returning to find Death Eaters have breached Hogwarts through Draco's scheme. On the tower, Draco cannot bring himself to kill Dumbledore, and Snape does it instead, then flees, revealing that he was the Half-Blood Prince. The retrieved Horcrux proves a fake. Dumbledore is dead, and Harry resolves to leave school and hunt the remaining Horcruxes.

Harry Potter, orphaned when Lord Voldemort killed his parents and was defeated trying to kill him, learned at eleven that he is a wizard, went to Hogwarts, joined Gryffindor, and befriended Ron Weasley and Hermione Granger. Over five years he thwarted Voldemort's schemes again and again: stopping the theft of the Philosopher's Stone, destroying a diary holding the young Voldemort's memory, and clearing his godfather Sirius Black. Forced into a deadly tournament, he saw Voldemort restored to a body using his blood, and the following year, persecuted by a Ministry in denial, he formed a secret student defence group. That year ended at the Ministry of Magic, where Sirius was killed and Voldemort was at last seen publicly, forcing the world to admit he had returned. Harry learned a prophecy binding him to Voldemort: neither can live while the other survives. The war is now open, and Dumbledore, the headmaster, has begun preparing Harry for the confrontation that only one of them can survive.

The wizarding world is now openly at war, with Voldemort's followers spreading fear and death and the Ministry struggling to cope. In a shadowy scene, Snape makes an Unbreakable Vow to Draco Malfoy's mother, Narcissa, swearing to protect Draco and to complete a mysterious task himself if the boy fails. Dumbledore collects Harry from the Dursleys in person and takes him to recruit an old colleague, Horace Slughorn, back to Hogwarts as Potions teacher. Dumbledore reveals that he will give Harry private lessons this year, and that his own hand has been badly and mysteriously injured. On a subdued shopping trip to Diagon Alley, Harry secretly follows Draco to a shop selling dark objects and comes away convinced that Draco is up to something sinister, perhaps that he has become a Death Eater.

Term begins with Harry made Quidditch captain and unexpectedly excelling in Potions, thanks to a battered second-hand textbook crammed with the improved instructions and invented spells of a previous owner who signed himself the Half-Blood Prince. Dumbledore's private lessons start, and together he and Harry begin exploring memories of Voldemort's past: his birth to a poor, abandoned mother, and his cold, frightening childhood in an orphanage as the boy Tom Riddle, who was already cruel and drawn to power before Dumbledore ever brought him to Hogwarts. Dumbledore is clearly building toward some vital secret. He shows Harry a tampered memory belonging to Slughorn, from which a crucial piece has been removed, and asks Harry to use his growing closeness with the new teacher to recover the true version.

The sixth-years are caught up in romances and jealousies: Ron takes up with a clingy classmate, driving a wedge between him and Hermione, while Harry finds himself increasingly drawn to Ron's sister, Ginny. Beneath the ordinary school year, danger presses in. A student is gravely cursed by a necklace clearly meant for someone else, and Harry is certain Draco is behind it and is trying to reach Dumbledore, though the adults dismiss his suspicions. Dumbledore continues showing Harry memories of the young Voldemort, including one in which the teenage Riddle questioned Slughorn about a piece of very dark magic, the missing part of the memory Dumbledore still needs. Slughorn, ashamed, refuses to give up the true recollection, and Harry cannot yet coax it from him.

Danger keeps striking close: Ron is nearly killed by poisoned drink apparently intended for Dumbledore, further proof to Harry that someone inside the school is trying to murder the headmaster. During a confrontation in a bathroom Harry uses an unknown spell from the Prince's book against Draco and badly wounds him, horrified at what the scrawled curse does. Finally, dosing himself with a potion of pure luck, Harry succeeds where nothing else worked and draws the true memory out of Slughorn. It reveals Voldemort's terrible secret: he asked Slughorn about Horcruxes, objects into which a wizard can split his soul to escape death, and boasted of intending to make several. Dumbledore concludes that Voldemort made multiple Horcruxes, and that each must be found and destroyed before Voldemort himself can be killed.

Dumbledore takes Harry to a hidden seaside cave to retrieve one of the Horcruxes, a locket, forcing himself to drink a torturous potion to reach it and returning gravely weakened. They come back to find the Dark Mark over Hogwarts: Draco has smuggled Death Eaters into the castle through a repaired magical cabinet. On the astronomy tower Draco disarms Dumbledore and admits he was ordered to kill him, but cannot bring himself to do it. Snape arrives and kills Dumbledore, then flees the castle, revealing to Harry as he goes that he himself is the Half-Blood Prince. Worse, the locket they nearly died for is a fake, containing a note from a thief signed only 'R.A.B.,' meaning the real Horcrux is still out there. Dumbledore is buried on the grounds, the school's future in doubt. Harry breaks off his new relationship with Ginny to keep her safe and resolves not to return for a final year, but to seek out and destroy Voldemort's remaining Horcruxes; Ron and Hermione vow to go with him.

Half-Blood Prince ends with Dumbledore's death and Harry's turn toward the final fight. Returning from a mission to destroy one of Voldemort's Horcruxes, the soul-fragments that keep him from dying, Dumbledore and an exhausted Harry find the Dark Mark over Hogwarts: Draco Malfoy has smuggled Death Eaters into the castle through a repaired vanishing cabinet. On the astronomy tower Draco disarms Dumbledore but cannot make himself commit murder, and Snape arrives and kills Dumbledore, then escapes, revealing as he flees that he is the Half-Blood Prince whose old textbook Harry had relied on all year. The locket Dumbledore and Harry retrieved from the cave, at terrible cost, turns out to be a fake, left in place of the real Horcrux by an unknown thief who signs only 'R.A.B.' Dumbledore is buried at Hogwarts, and the school may not reopen. Harry ends his budding romance with Ginny to keep her out of danger, and, with Ron and Hermione pledging to come with him, decides not to return for a final year but to find and destroy Voldemort's remaining Horcruxes so that the Dark Lord can at last be killed. Voldemort is ascendant, Snape has fled to his side, and the war is entering its darkest phase.