The night before the planned bank robbery, everything is ready, down to their disguises.
Hermione found a long, black hair – one of Bellatrix's – stuck on her sweater from Malfoy Manor, which will enable them to use Polyjuice Potion to imitate her. So Hermione will turn into Bellatrix, and will even have her wand to make the disguise more convincing.
But Hermione's miserable; she misses her own wand. Everyone's jealous of Luna, because her friend Mr. Ollivander sent her a new wand that morning.
Griphook comes in and Harry goes over the plan – they'll leave the next morning. They've already told Bill and Fleur that they're leaving (but not what they're going to do), and that they won't be back.
Griphook is clearly suspicious of his human co-conspirators, and won't ever leave the trio alone. Harry's not sure how they're going to get away with taking the sword with them.
The next morning at six, they're ready to go. Hermione's transformed into Bellatrix, and Ron is disguised as a completely invented character, with various physical changes that Hermione's crafted – bet she had fun with that. Harry and Griphook will hide under the Invisibility Cloak.
They Disapparate and arrive at the Leaky Cauldron, then proceed into Diagon Alley. It's deserted, and many of its previously busy shops are boarded up and abandoned. The whole street is plastered with "Undesirable Number One" posters with Harry's face.
There are a few destitute wizards roaming around, begging for money and mercy, but they slink away as Hermione/Bellatrix approaches. One of them confronts her, but Ron Stuns him.
Travers, a Death Eater, shows up and decides to accompany them to Gringotts.
At the entrance to the bank, they encounter the first security measure – some guards with Probity Probes, which detect lies. Harry, hidden under the cloak, Confounds the guards, and in their confused state, they let the whole bunch in.
Hermione tells one of the goblin bankers that she wants to get into her vault. However, the goblins already know that Bellatrix's wand has been stolen, and that they're imposters. Acting on Griphook's quick-witted guidance, Harry uses the forbidden Imperius Curse, first on the goblin, then on Travers.
The goblin trots off obediently to get something called the Clankers, and Harry tries to reduce the amount of attention they're drawing to themselves, which is mighty tough. The goblins seem to be on to them.
They've made it into the inner sanctum of the bank, though, so they go on. Using Bogrod, the goblin Harry Imperiused, they ride through the bank's labyrinthine tunnels in a cart, going at breakneck speed until they hit a magical waterfall. It undoes Ron and Hermione's illusory disguises – Griphook says this means that the goblins have set off the defense mechanisms. Uh oh…
The last obstacle is a half-blind, terrifying-looking dragon. Griphook tells them to shake the Clankers; the dragon is afraid of the sound, and backs away.
Finally, after making Bogrod press his palm to the vault door (the only key to a Gringotts vault), they're successfully in the Lestranges' vault.
Harry's already described Helga Hufflepuff's cup, the remaining Horcrux he knows about, to Ron and Hermione, but if it's something else… well, they'll cross that bridge when they come to it.
Hermione grabs a jeweled goblet – and, horrifyingly, there's a cascade of a ton of identical goblets.
Griphook informs them that the treasures are protected by curses that both make the items multiply so that the originals are lost in a sea of replicas, and that make the metal burning hot to the touch.
Thankfully, Harry spots Hufflepuff's cup up on a high shelf. He tries to avoid touching anything by reaching for its handle with Gryffindor's sword – and in a hurry, because they can hear a number of goblins approaching outside.
Hermione hoists Harry up into the air magically, but he bumps into a suit of armor, sending a cascade of burning copies down onto Ron, Hermione, and the goblins. They bump into more things, and soon there's a sea of scalding hot treasures and replicas. A hot mess!
Harry pulls Griphook up, who's almost swallowed up by the explosion of objects, and Griphook grabs onto the sword – the cup flies through the air, and Harry catches it, holding it despite the blistering heat, as they flee the vault.
Griphook immediately abandons them, taking the sword with him. He turns against the trio, joining the horde of goblins.
Harry blasts the dragon free of its shackles, and he, Ron, and Hermione clamber atop the creature. The dragon, realizing it's finally free, makes a break for it, with his passengers still hanging on.
The three of them cling on for dear life, and blast obstacles out of their ride's way. Finally, they get out of Gringotts and into Diagon Alley, and from there, to the sky.