Harry Potter and the Half-Blood Prince
Having returned to power, Death Eaters attack both the Muggle and Wizarding worlds, while Lord Voldemort has given Draco Malfoy an assignment. Bellatrix Lestrange goads Severus Snape into making an Unbreakable Vow with Draco's mother, Narcissa, to protect Draco and carry out the assignment if he fails. Scarred by his experience at the Ministry of Magic with Voldemort, Harry is reluctant to return to school. Dumbledore brings Harry to entice former Potions Professor Horace Slughorn to return to Hogwarts. Later, while leaving Fred and George's new shop in Diagon Alley, Harry, Ron and Hermione notice Draco associating with Bellatrix Lestrange, Fenrir Greyback and Narcissa Malfoy in Borgin and Burkes, leaving Harry suspicious. He attempts to eavesdrop on Malfoy on the Hogwarts Express, but Draco Petrifies Harry. He is rescued by Luna Lovegood and her Spectrespecs.
At Hogwarts, Harry and Ron borrow school textbooks for Slughorn's Potions class. The previous owner of Harry's copy, the "Half-Blood Prince", has annotated the book with additional instructions that allow Harry to excel in class and win a vial of the luck potion Felix Felicis. After making it as Keeper on the Quidditch team, Ron becomes a hero and he forms a relationship with Lavender Brown, which leaves Hermione heartbroken. After Harry finds her sobbing in a corridor, he confesses to having feelings for Ginny Weasley and admits that he is feeling distraught over her relationship with Dean Thomas. He hides his feelings from all but Hermione, however, knowing Ron is protective of Ginny and would never allow a relationship between them. Harry spends Christmas with the Weasleys, during which he discusses the situation at Hogwarts with Order of the Phoenix members Arthur Weasley, Remus Lupin and Nymphadora Tonks. He becomes closer to Ginny, who has broken up with Dean. Bellatrix Lestrange and Fenrir Greyback attack the Burrow and attempt to kidnap Harry. The Order is able to keep everyone safe, but the Death Eaters set fire to the Burrow.
Draco continues eluding Harry while perfecting the use of a Vanishing Cabinet inside the Room of Requirement. Harry suspects Draco is behind two attempts on Dumbledore's life, one of which nearly kills Ron. In his semi-comatose state, Ron mumbles Hermione's name, causing his relationship with Lavender to deteriorate. Confronting Draco, Harry hits him with a curse from the Half-Blood Prince's book which severely wounds him. Snape heals Draco as Harry retreats. Fearing the book may be filled with more Dark Magic, Ginny convinces Harry to leave the book in the Room of Requirement so that he won't use it again. While Harry's eyes are closed, Ginny hides the book and kisses him, starting a relationship between them.
During private meetings held throughout the year, Dumbledore shows Harry memories of a young Tom Riddle and reveals Slughorn retains a memory critical to Voldemort's defeat. Harry retrieves the memory using Felix Felicis. It reveals that Voldemort had been seeking information for creating as many as seven Horcruxes, devices that safeguard a portion of the creator's soul, granting him immortality unless the Horcruxes are destroyed. Two of Voldemort's Horcruxes have already been destroyed: Tom Riddle's diary, and his grandfather's ring. After locating another Horcrux, Dumbledore requests Harry's help to retrieve it. Inside a cave, Harry is forced to make Dumbledore drink a mind-altering potion that hides the Horcrux, a locket. Though gravely weakened, Dumbledore defends them from a horde of Inferi and Apparates himself and Harry back to the Astronomy Tower at Hogwarts.
Dumbledore first tells Harry to fetch Snape for help, but then tells him to hide when footsteps approach. Draco appears and reveals that Voldemort has chosen him to kill Dumbledore, but is unable to follow through. Snape arrives, motions to Harry to stay hidden, and joins the Death Eaters that arrive through the Vanishing Cabinet and surround Draco as he hesitates. Snape casts the Avada Kedavra curse, killing Dumbledore and then escaping from the castle with the other Death Eaters. In their wake they cast the Dark Mark, wreck the Great Hall and set fire to Hagrid's Hut. Harry tries to stop them, but Snape deflects Harry's spells and Bellatrix stuns him. Before departing, Snape reveals to Harry that he is the Half-Blood Prince. Harry returns to the school to find the staff and students mourning Dumbledore.
Harry reveals to Ron and Hermione that the locket Horcrux was a fake. The locket contains a message from an "R.A.B." stating he has taken the real Horcrux and hopes to destroy it and the others. Rather than return for their final year at Hogwarts, Harry and his friends vow to seek out R.A.B. and the remaining Horcruxes as Dumbledore's phoenix, Fawkes, flies into the horizon.