They land on Hogsmeade's High Street, and a screeching alarm immediately goes off. A gaggle of Death Eaters explodes out of the door of the Three Broomsticks pub, and the first tries to summon Harry's Invisibility Cloak. Amazingly, it doesn't respond to the spell. (That's right – Cloak with a capital "C.")
The Death Eaters are sure that the trio is there, though. They scatter rapidly, looking for Harry.
Meanwhile, the three of them try to Disapparate, but the Death Eaters have cast spells to keep them there – and furthermore, they've called in dementors. Harry, Ron, and Hermione can feel the characteristic cold settle over them.
In desperation, Harry releases his Patronus to get rid of the dementors. As he does, the Death Eaters see it and come after him, but before they get there, a door opens and someone urgently whispers for Harry to come in.
Harry, Ron, and Hermione find themselves in grimy old Hog's Head. They run upstairs, to an apartment with a painting of a spacey looking blond girl.
Downstairs, they can see a confrontation taking place between their savior, the barman of the Hog's Head, and the angry crowd of Death Eaters.
The barman lies to the Death Eaters convincingly, saying that he set off the charmed alarm by putting his cat out for the night, and that the Patronus they saw was his (a goat), not Harry's stag. Now that's a man who thinks on his feet.
The Death Eaters, disgruntled, depart.
Meanwhile, upstairs, Harry notices something odd – a small mirror propped up on the mantelpiece, below the portrait of the girl.
The barman comes back, and Harry immediately knows that it's this guy's brilliant blue eyes that he's been seeing in his side of the magic mirror.
He's the one who sent Dobby and is, of course, saddened by the news of Dobby's death.
But wait – more than that, he's Aberforth Dumbledore, Albus's famously odd younger brother.
Aberforth explains that he'd bought the other side of the mirror from Mundungus Fletcher, and Albus explained to him what it did. He's been keeping an eye on Harry ever since.
For a moment, Ron is elated – did Aberforth send the doe Patronus to them? Aberforth not-so-gently reminds him that they just saw his Patronus, a goat. That mystery remains mysterious for the time being.
Aberforth gives them some much-needed food, and encourages them to sneak out at daybreak when the curfew lifts – he wants them to go hide in the mountains where Hagrid is, to be safe.
Harry firmly refuses to escape. He tells Aberforth that Albus, his brother, left him a mission and he has to do it.
Aberforth bitterly tells Harry that he doesn't owe Albus anything, and that he should save himself before the elder Dumbledore's schemes get him killed.
Aberforth has lost hope – he used to be a member of the Order of the Phoenix, but he's given up now, and thinks the best anyone can do is stay alive.
Aberforth explains why he is so doubtful of his brother's plans, and finally we get the truth about Dumbledore. Aberforth has reason for worrying that everyone Albus ever cared about gets hurt, and thinking that Harry will be the same…
Aberforth's story is as follows:
When they were kids, Ariana (Albus and Aberforth's sister) was attacked by some Muggle boys while innocently experimenting with magic. She was permanently traumatized, and was driven mad by her own magical nature.
The Dumbledores' father went after the kids who attacked his daughter, and was locked up in Azkaban for it. He never told the Ministry why he did it, because he was afraid they would take Ariana away and lock her in St. Mungo's Hospital for Magical Maladies.
Albus, Aberforth, and Kendra (their mother) had to be very secretive to keep anyone from finding out about Ariana, who might have been seen as a threat. They moved houses and tried to keep a low profile in Godric's Hollow.
Aberforth cared for his sister more than Albus, who was wrapped up in schoolwork.
When Ariana was fourteen, she accidentally killed her mother with an uncontrolled burst of her undisciplined magical power. This is why Albus canceled his round-the-world trip and came home – he wanted Aberforth to finish his education at Hogwarts, and someone had to care for Ariana.
Young Albus did well for a little while. That is, until he made a new friend, Gellert Grindelwald.
That summer, Aberforth watched as his brother grew closer and closer to his new buddy, and as they made plans to travel the world looking for the Hallows and changing the world.
Aberforth got into a showdown with Grindelwald – the latter used the Cruciatus Curse on him, and Albus tried to stop him. All three of them were dueling, throwing curses everywhere, and somehow Ariana was killed in the midst of all the chaos.
Grindelwald escaped and fled the country, and Aberforth and Albus were left with the fallout.
Aberforth thought that Albus saw himself as free from his family after that, but Harry knows otherwise – he interrupts the story and tells Aberforth that when Albus drank poison the night he died, he was tormented by some unseen memory; Harry's certain that it was of Grindelwald's torture of his family.
Aberforth's story concluded, he pauses to consider this.
He and Harry argue about the idea of the greater good – Harry claims that sometimes it is more important than individual lives. He's prepared to finish Dumbledore's plans and eliminate Voldemort, even if it means losing his own life.
Harry tells Aberforth that no matter what, he's got to get into Hogwarts.
Aberforth thinks this over, gazing at Harry with the blue eyes that are so like his brother's, and then turns to address the portrait of Ariana. He tells her to go – do what?
She retreats into the depths of the painting, where a long tunnel seems to open up. Whoa… everyone except Aberforth is confused by what's going on.
Aberforth explains that this is the only remaining way to get into Hogwarts; the other secret passages have been guarded.
As he speaks, there's a light at the end of the painted tunnel. Ariana reappears, growing larger and larger as she walks towards them – and someone's with her.
And it's a familiar, much welcomed someone: their old friend, Neville Longbottom, looking battered and down on his luck. But he's definitely ecstatic that Harry has finally come.