In Harry Potter and the Half-Blood Prince, Harry becomes increasingly curious about Lord Voldemort's past and his rise to power after witnessing the return of his arch-nemesis at the end of the previous movie. Dumbledore, who has been Harry's mentor and confidant throughout the series, takes Harry under his wing and begins to teach him more about Voldemort's past.
Meanwhile, a new professor at Hogwarts, Horace Slughorn, takes a liking to Harry and invites him to join his exclusive club of students known as the Slug Club. Through Slughorn's and Dumbledore's guidance, Harry learns more about Tom Riddle, who would later become Voldemort.
As Harry delves deeper into Voldemort's past, he comes across an old textbook in his Potions class that belonged to someone who calls himself the Half-Blood Prince. The book is filled with spells and potion-making tips that prove to be extremely valuable to Harry, leading him to excel in his Potions class.
As the school year progresses, Harry and his friends Ron and Hermione become increasingly intertwined in a web of mystery surrounding Voldemort's plans for the wizarding world. They discover a dangerous and powerful potion that Voldemort wants to use to ensure his own immortality.
In the climax of the film, Dumbledore and Harry embark on a dangerous mission to find one of Voldemort's Horcruxes, an object that holds a piece of the Dark Lord's soul. They ultimately succeed in their mission, but not without a heartbreaking loss that puts Harry on the path to fulfilling his destiny as the Chosen One.