[edit] Plot overview
Voldemort and his Death Eaters openly wreak chaos throughout Britain. Following public outcry over mishandling the Voldemort situation, Cornelius Fudge is forced to resign. Rufus Scrimgeour is the new Minister for Magic. As a result, Arthur Weasley receives a promotion.
At his home in Spinner's End, Severus Snape receives a visit from Draco Malfoy's mother Narcissa and her sister, Bellatrix Lestrange. Narcissa swears Snape to an Unbreakable Vow, ensuring he will protect Draco and, should her son fail, complete his mission for the Dark Lord.
Albus Dumbledore arrives at the Dursleys and asks Harry Potter to accompany him to see retired professor Horace Slughorn. With unwitting help from Harry, Dumbledore persuades Slughorn to resume his old teaching position at Hogwarts. Harry then spends the remaining summer months at The Burrow with the Weasleys and Hermione, who is also visiting. To his family's dismay, Bill Weasley has become engaged to Fleur Delacour, the Beauxbatons champion in the Triwizard Tournament. Harry, Ron, and Hermione receive their O.W.L. results. Hermione receives high marks in all her subjects, but Ron and Harry fail Divination and History of Magic. They are also unable to take N.E.W.T.-level Potions because Snape only accepts "O" (Outstanding) grades. The course is a requirement to Harry's becoming an Auror.
As school begins, Snape is surprisingly announced as the new Defence Against the Dark Arts instructor while Slughorn takes his place as the Potions teacher. Since Slughorn only requires a minimum "E" grade (Exceeds Expectations) at O.W.L. to take his N.E.W.T.-level Potion classes, Harry and Ron are now able to sign up. Slughorn lends Harry and Ron old Potions textbooks. Harry's copy is marked as the property of "The Half-Blood Prince". The talented former owner's handwritten notes help Harry excel in the class. As a reward, Slughorn gives him a small vial of Felix Felicis, a good luck potion.
As the year progresses, more Death Eater attacks occur that may be linked to events at Hogswart. On the first Hogsmeade visit, Katie Bell, a Gryffindor student is injured when she is fored to carry a cursed necklace while under the Imperius Curse. Harry suspects Malfoy or another Death Eater is involved.
Ron begins dating Lavender Brown ? although it is mostly to spite Hermione, who, Ron learned, had kissed Viktor Krum. In turn, Hermione flirts with the egotistical Cormac McLaggen, but this backfires. Ron and Hermione's continual bickering causes Harry to fear they will never reconcile. But when Ron is accidentally poisoned drinking meade that was intended for Dumbledore, Hermione is so distraught that she and Ron end their feud, and Ron soon breaks up with Lavender.
Dumbledore gives Harry private lessons using his Pensieve to view collected memories about Voldemort's past. A memory belonging to Slughorn is partially missing. Aided by the Felix Felicis potion, Harry retrieves it from him. Dumbledore speculates that Voldemort has split his soul into seven fragments, storing six pieces in Horcruxes to grant himself immortality, while leaving the seventh in his own body. Two Horcruxes have been destroyed (Tom Riddle's diary by Harry[HP2] and Marvolo Gaunt's ring by Dumbledore).
When Harry discovers Malfoy sobbing in Moaning Myrtle's bathroom, they begin hurling curses at each other. Harry casts Sectumsempra, causing blood to gush from Malfoy's body. Snape arrives and tends to Malfoy. Recognizing the spell, he orders Harry to hand over his Potions book. Harry instead gives him Ron's copy. Snape assigns Harry detention for the same day as the Quidditch finals. After detention, Harry learns that Gryffindor won the finals, with Ginny Weasley as their substitute Seeker. During the victory celebration, Harry's suppressed feelings for Ginny are revealed when he spontaneously kisses her. They soon begin a relationship.
Just before Harry and Dumbledore leave to find another Horcrux (Salazar Slytherin's locket), Harry gives the remaining Felix Felicis to Ron, Hermione and Ginny. He has suspected all year that Malfoy may be a Death Eater and has confided his suspicions to Dumbledore, who seemed unconcerned. He learns later that Dumbledore had Snape investigate. Believing Malfoy is involved in something sinister, Harry asks Ron, Hermione, and Ginny to patrol the halls. Harry then disapparates with Dumbledore to a secret cave. Upon retrieving the Horcrux, Dumbledore is seriously weakened by a potion he drank in order to uncover the locket inside a basin.
Returning to Hogsmeade, Harry and Dumbledore see Lord Voldemort's Dark Mark hovering over Hogwarts. They borrow broomsticks from Madam Rosmerta, whom they later discover to be under the Imperius Curse. They fly to the Astronomy Tower where they are ambushed by Draco Malfoy. Dumbledore paralyses Harry, who is under his Invisibility Cloak, just before Draco disarms Dumbledore. Draco reveals that he helped the Death Eaters enter Hogwarts, although Dumbledore discerns that the obviously frightened boy was coerced into aiding Voldemort's followers.
Death Eaters appear and urge Draco to fulfill his mission?killing Dumbledore?but Draco is reluctant. Snape arrives; Dumbledore uncharacteristically pleads with Snape. Hesitating, Snape kills Dumbledore with the Avada Kedavra killing curse. The spell's force hurls Dumbledore's body over the tower wall. Upon Dumbledore's death, Harry is freed from the paralysing spell. The Death Eaters flee, and Harry pursues Snape, who identifies himself as the Half-Blood Prince in a short-lived duel before escaping with Malfoy.
Harry recovers the locket from Dumbledore's body only to discover that it is a fake. Inside is a note from someone with the initials "R.A.B." who has stolen the real Horcrux and has vowed that it will be destroyed with the hope that when Voldemort meets his match he "will be mortal once more".
The school year ends abruptly with Dumbledore's funeral. Professor McGonagall is appointed Hogwarts' interim headmistress, although the school may never reopen. Regardless, Harry decides to leave Hogwarts to search for the remaining Horcruxes. Ron and Hermione vow to accompany him, while Harry ends his relationship with Ginny to protect her from Voldemort. The book concludes as Harry looks forward to Bill and Fleur's wedding and being comforted that "...there was still one last golden day of peace left to enjoy with Ron and Hermione".