The adventure primarily begins when Harry realizes he is allowed to take advanced Potions and has not yet bought a book for the class. When he begins using one of the old school books he discovers an inscription by The Half-Blood Prince, an anonymous old student who has re-invented potions and spells. Harry uses the book throughout the novel, eventually discovering that the previous owner might have been more nefarious than studious. This is another instance where maybe books shouldn’t be trusted when the additions are written by hand especially when they’re leading one to cheat in classes.
Rowling also takes the opportunity to develop Voldemort’s character even more fully. Harry learns about the villain's sad past and how he may have worked some of the most disturbing and powerful Dark Magic ever. It’s up to Harry to gain a new professor’s trust and learn all he can about Voldemort’s dark history and magics.
Harry learns more and more about Voldemort and his role in what everyone knows will be the ultimate showdown. He works closely with Dumbledore, who also reminds him to trust in his friends. But in a heart-wrenching ending the school is attacked by Death Eaters and not everyone gets out alive, putting into motion the harrowing ending to the Harry Potter series in the seventh and final book, Harry Potter and the Deathly Hallows.