This book picks up with the wizarding and muggle world in chaos, so much that the minister of magic and the prime minister are meeting up to discuss something that is clearly becoming a war. Many Wizards feel that even Hogwarts is not safe.

While visiting Diagon Alley to buy school supplies, Harry sees Draco Malfoy acting strangely, so Harry follows him, leading towards an assumption that Draco has become a Death Eater. Very few people believe Harry's claims.

Now Snape is no longer a defence against the dark arts teacher.

His replacement is professor Slughorn, as well as, he finds an annotated textbook that had been once used by someone who called themself `` The Half Blood Price´´.

Harry and Dumbledore have been working on private lessons of memory reading going into the past of Tom Riddle, although they needed one memory in particular to find out why Voldomort is so powerful, but unfortunately they only have access to an edited version of it, so what they need is Profesor Slughoern's potions to give up the original unedited memory because it is his, so Harry is given the challenge to get this memory.

Once Slughorn fins out that Harry is doing this, he avoids him, Harry drinks a potion called liquid luck which makes the consumer lucky for a certain amount of time. With the little time he has, Harry takes advantage of Slughorn manipulating his way to get the unedited memory. What they found out was that Slughorn had taught Tom what a Horcrox was, a Horcrox is basically that you can split your soul into multiple parts so that if one part of your soul dies, you can come back from one life to another.

Harry and Dumbledore take it apothemselfs to find and destroy all of them.

Returning from one of their journeys, they arrive at Hogwarts to find the dark mark in the sky invaded by Death eaters. Harry and Dumbledore go to a high tower, a trap is released on Dumbledore and he uses Magic to hide Harry so he can not intervene.

Draco comes out with orders from Voldomort to kill Dumbledore, he is shaking resistantly because he doesn't have the courage to do it, but more death eaters come and instigate him to kill Dumbledore. Finally it is clear that Draco can not do it, so Snape steps in and kills him himself. When Harry goes downstairs to find help he finds a war between Death Eaters and Wizards.

The profesors meet after the battle to make an arrangement to shut down the school for good as long as Voldomort is a threat.

Dumbledore was to be buried on school grounds, a bunch of wizards from all over the world attended to.

At the end of the book, Harry pledges to find all the Horcruxes and destroy them, along with Ron and Hermine.