Becky Sharp and Amelia Sedley have just completed their studies at Miss Pinkerton's Academy for Young Ladies, and are departing for Amelia's house in Russell Square. Becky is a strong-willed and cunning young woman determined to make her way in society, and Amelia Sedley as a good-natured, lovable though simple-minded young girl. At Russell Square, Miss Sharp meets the dashing and self-obsessed Captain George Osborne (Amelia's betrothed) and Amelia's brother Joseph ("Jos") Sedley, a clumsy and vainglorious but rich civil servant home from the East India Company. Hoping to marry Sedley, the richest young man she has met, Becky entices him, but she fails.
Becky Sharp says farewell to the Sedley family and enters the service of the crude and profligate baronet Sir Pitt Crawley, who has engaged her as a governess to his daughters. Her behaviour at Sir Pitt's house gains his favour, and after the premature death of his second wife, he proposes marriage to her. Then he finds she is already secretly married to his second son, Rawdon Crawley. Sir Pitt's elder half sister, the spinster Miss Crawley, is very rich, having inherited her mother's fortune of £70,000. How she will bequeath her wealth is a source of constant conflict within the Crawley family. Initially her favourite is Captain Rawdon Crawley. Becky acts as Miss Crawley's companion, supplanting the loyal Miss Briggs to establish herself in favour before breaking the news of her elopement with Rawdon. The misalliance so enrages Miss Crawley that she disinherits her nephew in favour of his elder brother, Pitt Crawley, the younger. The couple constantly attempts to reconcile with Miss Crawley, but she will see only her nephew and refuses to change her will.
While Becky Sharp is rising in the world, Amelia's father, John Sedley, is bankrupted. The close relationship between the Sedleys and Osbornes disintegrates after the Sedleys are financially ruined, and the marriage of Amelia and George is forbidden. George decides to marry Amelia against his father's will, pressured by his friend Dobbin, and George is consequently disinherited. As Napoleon has escaped his exile, the Napoleonic Wars began again. George Osborne and William Dobbin are deployed to Brussels, but not before an encounter with Becky and Captain Crawley at Brighton. Already, the newly wedded Osborne is growing tired of Amelia, and he becomes increasingly attracted to Becky who encourages his advances. At a ball in Brussels, based on the Duchess of Richmond's 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. The morning after their reconciliation, he is sent to Waterloo with Captain Crawley and Dobbin. Becky is indifferent to her husband's departure. She tries to console Amelia, but Amelia responds angrily. Becky resents this snub and a rift develops between the two women that lasts for years. She makes a profit selling her carriage and horses at inflated prices to the panicked Joseph Sedley, seeking to flee Brussels.
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 young George's godfather, gradually expresses his love for Amelia by small kindnesses toward her and her son. She is too much in love with George's memory to return Dobbin's affections. Saddened, he goes to India for many years. Meanwhile, Becky also has a son, also named after his father, but unlike Amelia, Becky is a cold, distant mother. She continues her ascent first in post-war Paris and then in London where she is patronised by the great Marquis of Steyne, who covertly subsidises her and introduces her to London society. Her success is unstoppable, and she is eventually presented at court to the Prince Regent himself.
Becky and Rawdon appear to be financially successful. Rawdon gambles heavily and earns money as a billiards shark. Becky accepts trinkets and money from her many admirers and sells some for cash. She borrows heavily from the people around her and seldom pays bills. The couple lives mostly on credit. While Rawdon seems unaware of the effect of his borrowing on the people around him, Becky is fully aware that her failure to pay bills bankrupts at least two innocent people: her servant, Briggs, whose life savings Becky borrows and fritters away, and her landlord Raggles, who was formerly a butler to the Crawley family. He invests his life savings in the townhouse that Becky and Rawdon rent. She also cheats innkeepers, milliners, dressmakers, grocers, and others who do business on credit. She and Rawdon obtain credit by tricking shops into believing they are receiving money from others. Sometimes, Becky and Rawdon buy time from their creditors by suggesting Rawdon received money in Miss Crawley's will or are being paid a stipend by Sir Pitt. Becky is carrying on an extramarital affair with the Marquis of Steyne.
At the summit of her social success, Becky's pecuniary relationship with the rich and powerful Marquis of Steyne is discovered after Rawdon is arrested for debt. Rawdon's brother's wife, Lady Jane, bails him out and Rawdon surprises Becky and Steyne in a compromising moment. Rawdon leaves his wife and through the offices of the Marquis of Steyne is made Governor of Coventry Island to get him out of the way, but Rawdon challenges the elderly marquis to a duel. Becky, having lost both husband and credibility, is warned by Steyne to leave the country; she wanders the continent, leaving her son in the care of Pitt Crawley and Lady Jane. Wherever Becky goes, she is followed by the shadow of the Marquis of Steyne. No sooner does she establish herself in polite society than someone turns up who knows her disreputable history and spreads rumours; Steyne himself hounds her out of Rome.
As Amelia's adored son George grows up, his paternal grandfather relents and takes him from the impoverished Amelia, who knows the rich and bitter old man will give him a better start in life than she could manage. After twelve years abroad, both Joseph Sedley and Dobbin return. Dobbin professes his unchanged love to Amelia.Amelia is affectionate, but she cannot forget the memory of her dead husband. Dobbin also becomes close to and good influence on young George. While in England, Dobbin mediates a reconciliation between Amelia and her father-in-law. The death of Amelia's father prevents their meeting. Osborne dies soon after; he had amended his will, bequeathing 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 the decrease in her annuity.
After the death of Mr Osborne, Amelia, Joseph, George and Dobbin go to Germany, where they encounter the destitute Becky. She meets the young George at a card table and enchants Joseph Sedley all over again. Becky has fallen in life. She is drinking heavily, has lost her singing voice and much of her looks and spends time with card sharps and con artists. Following Joseph's entreaties, Amelia agrees to a reconciliation when she hears that Becky's ties with her son have been 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 Amelia's late husband had given her, asking her to run away with him. This destroys Amelia's image of George, but not before Amelia has sent a note to Dobbin professing her love.
Becky gains control over Joseph Sedley. He dies of a suspicious ailment after signing a portion of his money to Becky as life insurance, setting her up with an income. Rawdon dies weeks before his older brother, whose son has already died; the baronetcy descends to Rawdon's son. Had he outlived his brother by a day, Becky would have become Lady Crawley legitimately, a title she uses anyway. Although Dobbin married Amelia and always treated her with great kindness, he does not fully regain the love that he once had for her. There is a final appearance for Becky, as cocky as ever, selling trinkets at a fair in aid of various charitable causes. She is living well again, as her son, the new baronet, has agreed to support her financially (in spite of her past neglect and indifference towards him); however, he declines any further relationship or communication.