The next morning, Harry wakes up before anyone else. He mulls over the mystery of Dumbledore – who was the man, really? What dark secrets was he hiding? What hadn't he told Harry?
Harry explores the rest of the house to take his mind off things, and ends up in Sirius's room, still full of his adolescent artifacts. Among them is a photograph of Sirius, Lupin, Peter Pettigrew, and James Potter.
He finds some papers scattered around. They include a letter to Sirius from Harry's mother, Lily, which relates the events of Harry's first birthday.
In her letter, Lily mentions Bathilda Bagshot (remember, we heard about her in the Rita Skeeter article – she apparently dished some dirt on Dumbledore).
Harry's overwhelmed by sentiment. He searches urgently for the rest of the letter, which was cut off, but can't find it. Instead, he finds a photograph of himself as a child, zooming around on a toy broomstick that was Sirius's birthday present to him.
Harry is interrupted by Ron and Hermione, who are in a panic because they woke up and couldn't find him. He shows them what he found.
Hermione notices that someone must have searched the room before they did. Harry thinks whoever it was must have been looking for the other half of the letter.
Since Bathilda Bagshot is still alive and living in Godric's Hollow, Harry wants to go there even more badly now. Hermione's reluctant, though – she's nervous because of how easily those Death Eaters found them yesterday.
They head back down to the kitchen, but on the way, Harry notices something – the other bedroom on the floor belonged to someone called Regulus Arcturus Black. Could this have been the R.A.B. who stole the locket Horcrux before Harry and Dumbledore got to it?
Regulus was apparently Sirius's Death Eater brother. The story suddenly falls into place – Regulus must have become disenchanted with Voldemort and turned against him.
They venture into the bedroom, which is decorated proudly in the Slytherin colors of green and silver. However, after searching the room, they can't find any trace of the real locket. Where on Earth is the Horcrux?
Hermione reminds them that it could be hidden anywhere in the house, remembering the various misadventures they had while trying to clean it out when it was still Order HQ.
Suddenly, she remembers – when they were cleaning two years ago, they did find a locket. They remember throwing it away, but Harry hopefully recalls that Kreacher, the Black family house-elf, stole tons of things from the rubbish pile.
The three of them run down to the kitchen to look through Kreacher's things. The locket is nowhere to be found.
Harry, who's officially the master of the house after Sirius left it to him in his will, calls for Kreacher, who's conveniently obliged to appear.
Turns out that Kreacher did indeed have the locket, but Mundungus Fletcher (a sneaky Order member) stole it.
Harry orders Kreacher to tell them everything he knows about the locket.
Kreacher's tale goes a little like this: once upon a time, Sirius's brother, Regulus, joined the Death Eaters. He told Kreacher that Voldemort needed a house-elf, and that he was volunteered for the task; he commanded Kreacher to do whatever Voldemort told him to, and then come home to Grimmauld Place.
Turns out Kreacher was sent to the same desolate island Harry and Dumbledore went in Book 6 to find the Horcrux. There, Voldemort forced Kreacher to drink all of the poisonous potion, then dropped the locket/Horcrux into the empty basin and refilled it with poison. Voldemort then heartlessly left Kreacher on the island.
To add insult to injury, Kreacher was pulled into the water by the Inferi (undead creatures) that filled it. He managed to escape by Disapparating back home (though Apparition for humans is impossible in the cave, house-elf magic is different – since Kreacher was ordered to come back home, he could).
Kreacher told Regulus what happened, and Regulus was clearly disturbed.
A little later, Regulus comes back to talk to Kreacher again. He asks Kreacher to take him to the island in the cave where he proceeds to drink all the potion, and has Kreacher swap out the real Horcrux for the fake locket that Harry and Dumbledore found years later. Kreacher is supposed to take the real locket home and destroy it.
He watches as his master drinks the poison, and then is pulled underwater by the Inferi.
At home, Kreacher tried to destroy the locket, but he couldn't (as we know, Horcruxes are almost impossible to destroy, so it's no wonder). Sirius and Regulus's mother is distraught by the disappearance of her "good" son, but Kreacher can't tell her what happened, since Regulus ordered him not to tell anyone.
Kreacher breaks down in sobs, his story told.
Harry can't understand how Kreacher could betray Sirius to Voldemort, when it was Voldemort who caused his beloved Regulus's death.
Hermione explains that this is what humans get for treating house-elves like slaves for so long (one of her pet peeves since the beginning of the series, as you'll recall).
Harry (very politely) asks Kreacher to go and fetch Mundungus Fletcher for them, so they can ask him where the locket is. To encourage Kreacher, Harry gives him the fake Horcrux locket; Kreacher is completely overwhelmed by gratitude and emotion. He shows some respect to Harry, Ron, and even Hermione for the first time before Disapparating to find Mundungus.