Harry's sixth year at Hogwarts opens to find him more mellow and grown up than ever. The death of Sirius Black has left an indelible mark on him, and he's more determined than ever to put an end to Voldemort and his Death Eaters. He's happy to escape the tyranny of the Dursley's early in the summer when Dumbledore picks him up to attend to a mysterious errand, which ends up in Harry's persuading ex-professor, Horace Slughorn, to come out of retirement to teach at Hogwarts again.
When the day finally arrives, Harry is as usual overjoyed to be back at school. He's been made Quidditch Team Captain. However, Harry and his friends are dismayed to find out that Professor Severus Snape has finally achieved his burning desire to become Defense Against the Dark Arts professor. Harry is excited to be taking private lessons with Dumbledore this year, who is showing him all about Voldemort's past. Dumbledore hopes that knowing as much as he can about Voldemort will help Harry in his final battle with the Dark Lord.
A mysterious book previously owned by the Half-Blood Prince comes into Harry's hands early on during his Potions class and changes the course of his school year by giving him a reputation for Potions brilliance that baffles everyone. Harry quickly grows to depend on the information he gets from the Half-Blood Prince's book and uses some of his spells outside of class, much to Hermione's displeasure.
Harry also keeps an increasingly close eye on Draco Malfoy. Convinced that Malfoy has replaced his father as the head of the Death Eaters, Harry believes Malfoy is constructing a dark plot inside the school but can't for the life of him figure out what it is. Between Ron and Hermione's skepticism, and Malfoy's disappearances from the school grounds, Harry has his hands full trying to uncover what Malfoy's up to.
As the year speeds by, Harry and Ron are both amazed at their blossoming love lives. Harry especially falls hard for someone he knows he shouldn't, his best friend Ron's sister, Ginny. His lessons with Dumbledore continue sporadically, and he's excited to hear that he might be able to go with Dumbledore on a dangerous mission to help destroy a horcrux that contains a part of Voldemort's soul.