Amy takes care of Arthur, who is slowly recovering from his illness in jail.
She keeps playfully offering him half of her fortune. He says no every time.
Finally she asks him if he wants to know how much her fortune actually is.
Ready? It's... nothing at all.
Dorrit had given all his money to Merdle to invest, so there is nothing left.
Arthur laughs and agrees now to share half of her fortune. She makes a nice little speech half-proposing to him and half-accepting a marriage proposal.
At that moment Flora and Mr. F's Aunt come up the stairs. Amy tells them the good news that they are engaged.
Flora asks Amy to go with her to a nearby café, where Flora makes a long flowery, nonsensical speech basically wishing them well and saying no hard feelings. Mr. F's Aunt, meanwhile, demands that Arthur be brought "for'ard, and I'll chuck him out o' winder!" (2.34.48).
In a little while, Meagles comes to visit Arthur in jail for the first time.
Meagles has news that Doyce is doing swimmingly in Russia. They love him over there and are psyched to have all of his inventions. They've given him medals and titles and honors, and he's made a heap of money.
Then Meagles says, actually, here he is!
Doyce is not at all mad at Arthur and says that mistakes are part of the learning process.
Also, Meagles and Doyce tell Amy and Arthur that everything has been taken care of and that Arthur is free to go. He can have his old job back at Doyce & Clennam and everything will be just fine.
Arthur and Amy decide to leave the next day, because first they want to... get married!
The next morning Amy brings Arthur a piece of paper to burn in the fire before they go to church. Arthur doesn't know what it is, but we can assume that it's the codicil to the will that leaves Amy all that money. She decides to tell him about his own family when the time comes, but not about the money.
They go to the same church where Amy spent the night with Maggy way back at the beginning of the book.
They get married.
And then they live quietly and peacefully ever after.