testHarry Potter and his best friends, Ron Weasley and Hermione Granger, return to Hogwarts School of Witchcraft and Wizardry for their sixth year of magical education. Before Returning they find out that Emmeline Vance and Amelia Susan Bones are murdered by Lord Voldemort. It is announced that Severus Snape has become the new Defence Against the Dark Arts instructor, while Horace Slughorn has taken Snape's place as Potions teacher. Harry discovers that the previous owner of his Potions textbook, the "Half-Blood Prince", has annotated the book with refinements that allow Harry to excel in class and become a favourite of Slughorn's. Slughorn is also intrigued by the rumour that Harry is the "Chosen One" who will finally kill the evil Lord Voldemort, who has recently regained power and is set on conquering the wizarding world.
Harry recognises his attraction to Ginny Weasley, but fears that acting on it will harm his friendship with Ron, her overprotective older brother. Ron begins dating Lavender Brown, causing a rift between him and Hermione, who secretly harbours feelings for him. The rift heals only when Ron is nearly killed by poisoned mead intended for Hogwarts' headmaster, Albus Dumbledore. Harry suspects that his nemesis, Draco Malfoy, has become one of Voldemort's supporters and believes he was behind both the mead and a previous failed attack on Dumbledore's life. However, no one seems to believe him.
During private meetings held throughout the year, Dumbledore uses his Pensieve to show Harry memories of Voldemort's past. A memory Harry procures from Slughorn confirms Dumbledore's suspicion that Voldemort splintered his soul into seven fragments in order to achieve near immortality. Six of these fragments are contained in magical objects called Horcruxes, which must be destroyed before Voldemort can be killed. Two Horcruxes have already been destroyed: Tom Riddle's diary, which Harry stabbed with a basilisk fang in his second year, and Voldemort's