Harry 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. 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.
