The main conflict is between Harry and Draco. Harry believes Draco has joined up with Voldemort, a very powerful, evil, wizard, and is planning to attack the wizarding school. Harry believes that Professor Snape is helping Draco do this, but Harry cannot get anyone to believe him. He tracks Draco and Snape to try and prove that they are planning something.
Dumbledore, the headmaster, gives Harry private magic lessons. He teaches Harry about Voldemort's past and other things he may need to know so that one day he can defeat Voldemort. Before these lessons, Harry confides in Dumbledore all the things he has noticed out of the ordinary. Harry tells him what he suspects of Snape and Draco but he will not believe him. However, Harry continues to track Draco.
Dumbledore takes Harry to a remote cave where he suspects Voldemort hid a piece of his soul, a Horcrux. Before Harry leaves with Dumbledore, he warns his friends that Draco may follow through with his plan while he and the headmaster are gone, because there will be nobody there to stop him. At the cave, they find the Horcrux, a locket, they were looking for, along with many obstacles which leave Dumbledore very weak and injured.
Harry and Dumbledore return to the school, with the locket, to find that Harry was right and Draco was planning to do something while they were gone. Draco brought some of Voldemort's followers into the castle and was planning to kill Dumbledore. However, Draco couldn't follow through with the murder so professor Snape came and killed Dumbledore instead.
After Snape kills Dumbledore, Harry goes into the castle to see a battle between the Death Eaters and his friends. The battle ends with no clear winner, but when Harry goes to look at Dumbledore's body he finds that the locket is still with Dumbledore. Harry opens the locket to find a letter that says it was a fake and someone else had already taken it before him. The letter is signed R.A.B, whom Harry does not know.
I have taken the Horcrux and will try and destroy it R.A.B
A funeral is held for Dumbledore and many people, human and creature, come to it. After the funeral, Harry tells his friends that he will not come back to school the following year and will instead finish finding all of Voldemort's Horcruxes. His friends agree with him and decide to drop out of school as well, so they can help him on his journey.