Spoilers through Harry Potter and the Deathly Hallows Part 1.
R.A.B. is Regulus Arcturus Black, the brother of Sirius Black (Gary Oldman). Ron Weasley (Rupert Grint) finds the name on a door in the Black family house. Regulus swapped the Horcrux locket for a fake and left a note saying he meant to destroy the real one, so Albus Dumbledore (Michael Gambon) went through the cave’s defences for an object somebody else had already replaced.
Harry gets a message after Dumbledore has paid for the wrong locket
Harry Potter and the Half-Blood Prince
Dumbledore raises his wand amid the cave's fire. Warner Bros. Pictures
Dumbledore drinks the potion in the cave, collapses under it and still has to drive the Inferi back with fire, all for a fake. After his death, the locket turns out to hold a note for Lord Voldemort (Ralph Fiennes), and Hermione Granger (Emma Watson) reads it out to Harry Potter (Daniel Radcliffe) on the Astronomy Tower. Its writer has “stolen the real Horcrux” and wants Voldemort “mortal once more”.
Dumbledore’s suffering has bought a clue and nothing more.
Harry has three initials to go on.
Ron finds Regulus in a house the trio already know
Harry Potter and the Deathly Hallows Part 1
Harry, Ron and Hermione arrive at the door of Grimmauld Place. Warner Bros. Pictures
After the wedding is attacked, and Death Eaters find them again in a café on Shaftesbury Avenue, the trio hide at Grimmauld Place. Ron stumbles on the answer there: a door with a name on it, Regulus Arcturus Black. Harry reads the note again, and the initials fit: “R.A.B. is Sirius’s brother.”
Kreacher remembers an order he could not carry out
Harry Potter and the Deathly Hallows Part 1
Mundungus faces Harry, Ron, Hermione, Dobby and Kreacher at Grimmauld Place. Warner Bros. Pictures
Shown the fake, Kreacher (Simon McBurney, voice) knows it at once: “It is Master Regulus’s locket!” Before Regulus died, “he ordered Kreacher to destroy the locket, but no matter how hard Kreacher tried, he could not do it.”
Kreacher and Dobby (Toby Jones, voice) drag in Mundungus Fletcher (Andy Linden), who stole valuables from the house. Mundungus says a “Ministry hag” asked to see his licence and would have locked him up, “if she didn’t take a fancy to that locket.” He picks her out of the newspaper: Dolores Umbridge (Imelda Staunton).
Voldemort’s prized secret has become Umbridge’s jewellery.
Ron finishes what Regulus set out to do
Harry Potter and the Deathly Hallows Part 1
Ron holds the Sword of Gryffindor beside the pool. Warner Bros. Pictures
Hermione’s Polyjuice Potion gets the three of them into the Ministry in disguise. In the courtroom where Umbridge is trying Muggle-borns, Harry turns her own lesson on her, “You are lying, Dolores. One must not tell lies,” then stuns her, and Hermione takes the locket from her neck.
Destroying it is the harder problem. They have no way to do it, and wearing it in turns makes them worse to each other, until Ron walks out. When he comes back, he pulls Harry out of a frozen pool, and the Sword of Gryffindor Harry was diving for becomes Ron’s weapon. The locket fights back, showing Ron a Harry and Hermione who say they were better off without him.
Ron brings the sword down anyway.
The real locket goes from a house-elf who could not destroy it, to a thief who stole it, to an official who took a fancy to it. Harry has to follow it through all three before Ron can finish it, while the cave’s grand defences guard a fake.
