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Hermione's library sleuthing has helped her get closer to discovering who the Half-Blood Prince is. She finds a newspaper article revealing that there was a witch at Hogwarts named Eileen Prince. Hermione says that maybe the Half-Blood Prince was a woman. Harry strongly disagrees.
Dumbledore summons Harry to his office, and along the way, Harry encounters Professor Trelawney who is a bit tipsy and a bit confused. She has a tendency to hide her sherry bottles in the Room of Requirement, but when she just recently entered the Room, there was someone else in there.
Harry is curious and suspects that the voice Trelawney heard in the Room belonged to Draco.
Trelawney tells Harry that the voice she heard seemed to be cheering; it seemed to be very happy.
Harry urges Trelawney to tell Dumbledore the story she has just related, and the two set off for Dumbledore's office.
Along the way, Trelawney relates the story of having told the prophecy of the Chosen One at her job interview in Hogsmeade. She tells Harry that Snape, of all people, happened to be eavesdropping on the other side of the door during her interview.
This sends Harry into a blind fury. He realizes that it was Snape who was responsible for delivering this prophecy to Voldemort, which led to the death of his parents.
Harry rushes off to Dumbledore's office, furious that Dumbledore has never before told him that Snape overheard the prophecy.
Dumbledore tells Harry that he has found a Horcrux and asks if Harry would like to come with him to retrieve it.
Harry tells him he would like to come, but anger at Snape is still boiling underneath the surface of Harry's words.
When Dumbledore asks Harry what is wrong, Harry tells him what he has just learned about Snape from Trelawney.
Dumbledore is quiet at first and then begins to explain to Harry the guilt Snape has since felt at having had a hand in the death of Harry's parents. He tells Harry that, yes, Snape worked for Voldemort at the time, but he had soon after left the Dark Lord. He also tells Harry that he trusts Snape completely.
Harry does not.
Things are tense in Dumbledore's office. He tells Harry to go retrieve his Invisibility Cloak and to meet him back in the office in five minutes. They will set off to find the Horcrux immediately.
As Harry packs up his things back in his dorm room, he gives Ron and Hermione the Marauder's Map and the good luck potion. He tells them to watch Draco carefully and to drink some of the potion (Ginny too) should any harm come to them while he is away.
Harry is clearly worried that Draco's joyful shout means that something terrible will happen tonight.