Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
Poor but resourceful orphan Becky and rich but passive Amelia set out from school into the real world.
This is a classic beginning to many Victorian novels: take two girls (check), usually a brunette and a blonde (check), whose personalities are markedly different (check). Then turn them loose to see how well they do. Since they are girls, and since this is Victorian times, that usually means "how well they marry."
Conflict
Becky realizes that she has to make things happen for herself. Amelia tries to conform to stereotype and waits for her Prince Not-So-Charming to come to her.
Young women are expected to just let life happen to them, rather than grabbing the bull by the horns. For Becky, this is a double bind, as Thackeray points out. On the one hand, Becky doesn't have anyone behind her to orchestrate a marriage, so she has to actively pursue her own options (Jos, for instance). On the other hand, being too aggressive opens her up to being called un-feminine and being seen as a sleazy gold-digger. Amelia is also in a double-bind. On the one hand, what makes her so likeable (we are told) is how soft and undemanding she is and how much she is willing to just accept whatever life gives her. On the other hand, this kind of femininity also makes her super-boring and taken for granted.
Complication
Becky marries Rawdon and thus misses out on getting either the Crawley fortune or the Crawley estate and title. Amelia marries George and thus gets a husband who doesn't respect her and a father-in-law who cuts off their allowance.
The stories of the two girls sync up nicely, with some good contrasts. Both have made financially dumb marriages. But Becky is the loved one in her marriage, while Amelia is the one who is doing the loving. Becky is still scheming and plotting and trying to move forward in the world, while Amelia sits patiently at home waiting for George to come to his senses and start to appreciate her.
Climax
Becky soars and then plummets. First up, up, up, to the heights of society. Then down, down, down, to being suspected of adultery with Lord Steyne and being left by her husband. Amelia loses her husband to death by Napoleonic bullet, her family wealth to bankruptcy by Napoleonic invasion, and her son to adoption.
Again the two women's stories sync up somewhat, even though the paths they took to get to their lowest points are as different as can be. Still, at the climax of the novel, both Becky and Amelia have lost everything of value. Becky's respectability, high social status, and a chance at great financial success are gone. Gone too are Amelia's two loves, her comfortable lifestyle, and her self-respect and dignity.
Suspense
Will Becky get Rawdon to take her back? Will she make up with Lord Steyne? If not, what will she do? Will Amelia ever get over George? Will she wake up and smell the coffee about Dobbin and his never-ending love?
Becky is inexhaustible and never gives up. She is the novel's eternal optimist, especially about herself. As soon as Rawdon leaves her, she immediately sets off to get his brother Pitt to reconcile them. When that doesn't work, she bums around Europe until she runs into a still-furious Lord Steyne, and without skipping a beat she tries to see if he'll have her back. Amelia, meanwhile, sinks back down into mopey mode, sighing over her dead husband and ignoring the long-suffering Dobbin. She doesn't even have the mental energy to get past a two-month marriage that ended ten years earlier.
Denouement
Becky latches onto Jos and this time doesn't let him go until the bitter end. When she does let him go, it's only to get her hands on his life insurance policy. Amelia has George's would-be cheating revealed to her by Becky and is finally able to ditch that loser's memory and realize that she wants to be with Dobbin.
Again, a bit of a matchup here between Becky and Amelia. Both end up settling for a second-rate marriage (well, couple-ship, since Becky and Jos cannot actually get married). But still, both at last find what they had wanted from the very beginning. In Jos, Becky has financial security for life. In Dobbin, Amelia has a husband who will baby her and allow her to lead the kind of passive, boring existence she prefers.
Conclusion
Becky kills Jos! Or maybe she doesn't. It's unclear. Amelia and Dobbin are happily married! Or maybe he falls out of love with her. Who knows.
It's a mega-happy ending for all! Just kidding. Thackeray never lets the cat out of the bag, so we readers get to decide for ourselves. What do you think happened to Jos? Will Dobbin and Amelia still be relatively content in middle age?