As Lord Voldemort tightens his grip on both the Muggle and wizarding worlds, Hogwarts is no longer a safe haven. Harry suspects perils may even lie within the castle, but Dumbledore is more intent upon preparing him for the final battle fast approaching. Together they work to find the key to unlock Voldemorts defenses and to this end, Dumbledore recruits his old friend and colleague Horace Slughorn, whom he believes holds crucial information. Even as the decisive showdown looms, romance blossoms for Harry, Ron, Hermione and their classmates. Love is in the air, but danger lies ahead and Hogwarts may never be the same again.
In Harry Potter's (Daniel Radcliffe's) sixth year at Hogwarts School of Witchcraft, he finds a book marked mysteriously, "This book is the property of the Half Blood Prince", which helps him excel at Potions class and teaches him a few dark and dangerous ones along the way. Meanwhile, Harry is taking private lessons with Dumbledore (Sir Michael Gambon) in order to find out about Voldemort's (Ralph Fiennes') past so they can find out what might be his only weakness.