The story opens at Miss Pinkerton's Academy for Young Ladies, where the principal protagonists Becky Sharp and Amelia Sedley have just completed their studies and are preparing to depart for Amelia's house in Russell Square. Becky is portrayed as a strong-willed and cunning young woman determined to make her way in society, and Amelia Sedley is a good matured, lovable though simple-minded young girl.
At Russell Square, Miss Sharp is introduced to the dashing and self-obsessed Captain George Osborne (to whom Amelia has been betrothed from a very young age) and to Amelia's brother Joseph Sedley, a clumsy and vainglorious but rich civil-servant fresh from India. Becky entices him and hopes to marry him, though eventually fails as a result of warnings from Captain Osborne and his own native shyness and embarrassment that Becky had witnessed his foolish behavior at Vauxhall.
With this Becky Sharp says farewell to Sedley's family and enters the service of the baronet Sir Pitt Crawley who has engaged her as a governess to his daughters. Her behavior at Sir Pitt's house gains the favor of Sir Pitt, who after the premature death of his second wife, proposes to her. However, it soon transpires that she is already secretly married to his second son, Rawdon Crawley.
Sir Pitt's half sister, the spinster Miss Crawley, is very rich having inherited her mother's fortune of ￡70,000. Where she will leave her great wealth is a source of constant conflict between the branches of the Crawley family who vie shamelessly for her affections; initially her favorite is Sir Pitt's younger son, Captain Rawdon Crawley. For some time, Becky acts as Miss Crawley's companion, supplanting the loyal Briggs in an attempt to find favor before breaking the news of her elopement with her nephew. The misalliance so enrages Miss Crawley, that she eventually disinherits her nephew in favor of his elder brother, who also bears the name Pitt Crawley. The couple constantly attempt to reconcile with Miss Crawley and she relents a little. However, she will only see her nephew and refuses to change her will.
While Becky Sharp is rising in the world, Amelia's father, John Sedley, is bankrupted. The relationship between the two families, the Sedleys and Osbornes, who were once close allies disintegrates and the marriage of Amelia and George is forbidden. George ultimately decides to marry Amelia against his father's will, primarily due to the pressure of his friend Dobbin, and George is consequently disinherited by his father.
When all these personal incidents are going on, the Napoleonic Wars have been ramping up, and George Osborne and William Dobbin are suddenly deployed to Brussels, but not before an encounter with Becky and Captain Crawley at Brighton. The holiday is interrupted with orders to march to Brussels. Already, the newly wedded Osborne is growing tired of Amelia, and he becomes increasingly attracted to Becky.
At a ball in Brussels(based on the Duchess of Richmond's famous ball on the eve of the battle of Waterloo) George gives Becky a note inviting her to run away with him. He regrets this shortly afterwards, and reconciles with Amelia, who has been deeply hurt by his attentions towards her former friend. The morning after, he is sent to Waterloo, with Captain Crawley and Dobbin, leaving Amelia distraught. Becky, on the other hand, is virtually indifferent about her husband's departure. She tries to console Amelia, but Amelia responds angrily, disgusted by Becky's flirtatious behavior with George and her lack of concern about Captain Crawley. Becky resents this snub and a rift develops between the two women that lasts for years.
Captain Crawley survives, but George dies in the battle. Amelia bears him a posthumous son, who is also named George. She returns to live in genteel poverty with her parents. Meanwhile since the death of George, Dobbin, who is his son's godfather, gradually begins to express his love for the widowed Amelia by small gestures directed towards her and her son. Most notably is the recovery of an old piano, which Dobbin picks up at an auction following the Sedley's ruin, which Amelia mistakes as a gesture from her late husband. She is too much in love with George's memory to return Dobbin's affections. Saddened, he goes to India for many years. Dobbin's infatuation with Amelia is a theme which unifies the novel and one which many have compared to Thackeray's unrequited love for a friend's wife.
Meanwhile, Becky also has a son, also named after his father, but unlike Amelia, who dotes on and even spoils her child, Becky is a cold, distant mother. She continues her ascent first in post-war Paris and then in London where she is patronized by the great Marquess of Steyne who covertly subsidies her and introduces her to London society. Her success is unstoppable despite her humble origins and she is eventually presented at court to the Prince Regent himself.
At the summit of her success, Becky's pecuniary relationship with the rich and omnipotent Marquess of Steyne is discovered by Rawdon, after he is arrested for debt. His brother's wife, Lady Jane, bails him out and he surprises the couple in a compromising position. Rawdon leaves his wife and through the offices of Lord Steyne is made Governor of Coventry Island to get him out of the way, after Rawdon challenges the elderly marquess to a duel. Mrs Crawley, having lost both husband and credibility, is warned by Steyne to quit England and wanders the continent. Rawdon and Rebecca's son is left in the care of Pitt Crawley and Lady Jane. However wherever Becky goes, she is stalked by the shadow of Lord Steyne. No sooner has she established herself in polite society, than someone turns up who knows her disreputable history and spreads rumors; Steyne himself hounds her out of Rome.
As Amelia's adored son George grows up, his grandfather relents and takes him from poor Amelia who knows the rich and bitter old man will give him a much better start in life materially than she or her family could ever manage. After twelve years abroad both Joseph Sedley and William Dobbin return to England. Dobbin professes his unchanged love to Amelia, but although Amelia is affectionate to Dobbin, she tells him she cannot forget the memory of her dead husband. Dobbin also becomes close to George, and his kind firm manner with him proves a good influence on the spoil child.
While in England, Dobbin mediates a reconciliation between Amelia and her father-in-law. The death of Amelia's mother prevents their meeting but following Osborne's death soon after, it is revealed that he had amended his will and bequeathed young George half his large fortune and Amelia a generous annuity. The rest is divided between his daughters, Miss Osborne and Mrs Bullock who begrudges Amelia and her son for the decrease in her annuity.
After the death of old Mr Osborne, Amelia, Joseph, George and Dobbin go on a trip to Germany, where they encounter the destitute Becky. She meets the young George Osborne at a card table and then enchants Jos Sedley. Following Jos' entreaties, Amelia agrees to a reconciliation (when she hears that Becky has had her ties with her son severed), much to Dobbin's disapproval. Dobbin quarrels with Amelia, and finally realizes that he is wasting his love on a woman too shallow to return it.
However, Becky, in a moment of conscience, shows Amelia the note that George (Amelia's dead husband) had given her, asking her to run away with him. This breaks George's idealized image in Amelia's mind, but not before she has already sent a note to Dobbin professing her love.
Becky resumes her seduction of Joseph Sedley and gains control over him. He eventually dies of a suspicious ailment after signing a portion of his money to Becky as life insurance. In the original illustrations, which were done by Thackeray, Becky is shown behind a curtain with a phial (presumably of poison) in her hand; the picture is labelled 'Becky's second appearance in the character of Clytemnestra.' (She had played Clytemnestra during charades at a party earlier in the book.) His death appears to have made her fortune.
By a twist of fate Rawdon Crawley dies weeks before his elder brother whose son has already died. Thus the baronetcy descends to Rawdon's son. Had he outlived his brother by even a day he would have become Sir Rawdon Crawley and Becky would have become Lady Crawley - the title she uses regardless in later life.
