Harry Potter's sixth year at Hogwarts turns out to be quite the exciting year. First off is the arrival of a new teacher at Hogwarts, Horace Slughorn, who is a bit more useful to Harry than he realizes. Next, Harry obtains a Potions book which used to belong to the very mysterious Half-Blood Prince. Harry finds that the Half-Blood Prince's ancient scribbles are written along the margins of almost every page, giving Harry advice on how to improve greatly on his Potions work, and also teaching him a few helpful (and dangerous) spells along the way.

Amidst this, Harry is starting private lessons with Professor Dumbledore, during which Harry learns the dark secrets of Voldemort's past, hoping that they could use these secrets to find a way to defeat him.

Harry's year gets even more stressful with the suspicious actions of Draco Malfoy, who has been sneaking around the school doing, so Harry assumes, Voldemort's bidding. Harry quickly becomes determined, and slightly obsessed, to find out exactly what Malfoy has been up to and putting an end to it.

Yet, during this time, Harry and his friends go through daily life, busy with school work, Quidditch (in which Harry has been made captain of the team), and, of course, romance. Ron has found a new girlfriend, Lavender Brown, a perky (if not obnoxious) Gryffindor student, and Hermione is not happy about it. Ron and Hermione's friendship takes a toll throughout the school year and Harry, as usual, is stuck in the middle. Harry, meanwhile, is facing a romantic dilemma of his own: he realizes he is falling for his best friend's sister, Ginny Weasley, who is unfortunately dating Harry's classmate, Dean Thomas. Harry's pining for Ginny and Ron's hilarious relationship with Lavender give this story a large dose of reality.

Throughout all the school drama, however, the obvious darkness of Voldemort's impending rise to power is always apparent. The incredible action-packed climax is sure to leave the audience stunned and, inevitably, prove that you shouldn't trust everybody who you think is good and also prove that not everyone can manage to survive.