As Michael is tortured by the thought that Fleur may be interested in Wilfred, he encounters June, who lets slip some family history: Fleur was on the rebound from her first great love when she married Michael. All negative thoughts are buried, however, when at long last, Fleur becomes pregnant. Elderson cans Butterfield for confiding in Soames, who feels guilty enough to secure a new job for Butterfield at Michael’s firm. Meanwhile, Victorine’s modeling has brought in enough money for the Bickets to immigrate to Australia, but not before Tony sees the nude portrait of his wife hanging in a gallery.

Twenty years have passed (the Great War slips by here) as the third generation of Forsytes takes center stage. Jon has grown into an earnest and naive young man who decides to train as a farmer at Holly and Val’s horse farm in Surrey. Fleur has become an impetuous, beautiful, thoroughly spoiled young woman. A chance meeting at June’s art gallery sets the stage for the relationship which will affect the remainder of the series. Fleur manipulates a visit to Holly and Val’s, where she and Jon become infatuated. Aware that a feud exists between their families, but ignorant of the specifics, they promise to hold onto their love forever.

The elder Forsytes continue to hide the details of the family feud from Jon and Fleur. Michael Mont, the perky son of a baronet, courts Fleur and gains the respect of Soames, while Jo hides his ill health from Irene. Belgian profligate Profond discovers the truth behind the feud, courtesy of Forsyte cousin George. He begins an affair with Annette, and drops a bombshell on Fleur: her father was once married to Jon’s mother.

The annual cricket match between Eton and Harrow facilitates awkward encounters. Fleur scrambles to get Jon to the altar before he learns the truth behind the family feud. Before he dies, Jo finally tells Jon the entire story of his mother’s first marriage, her adultery, and her rape. Jon chooses Irene over Fleur, and they plan to move abroad, as Fleur, rejected and hurt, finds comfort from Michael Mont.

Jon and Irene relocate to Canada. On the rebound, Fleur marries Michael Mont, who assumes narration of the series. Two years and more pass, during which a new cast of characters arrives on the scene. Michael’s father Sir Lawrence (“Bart”) invites Soames to join the board of directors of an insurance company, where Soames is suspicious of the managing director, Elderson. Poor guttersnipe Bicket is sacked from his position at Michael’s publishing house, and is forced to sell balloons on the street to support his ailing wife, Victorine. Fleur flirts dangerously with Michael’s best friend, moody poet Wilfred Desert.

Wilfred confesses his love of Fleur to Michael, as Soames lobbies for a grandchild. The board of directors is slow to act, while Elderson’s subordinate Butterfield confirms Soames’s suspicions. Without her husband’s knowledge, Victorine Bicket agrees to pose nude for artist Aubrey Greene.

Both Michael and Tony address issues of trust in their marriages. The Bickets leave the squalor of their London slum in favor of a new life in Australia. Elderson admits his larceny, then flees the country, leaving Soames and Bart facing charges at a contentious stock holders’ meeting. Holly makes a tentative move of friendship toward Fleur, who gives birth to a son, Kit.

The scene shifts briefly to the Carolinas, where Jon and Irene are guests of young Ann Wilmot and her brother Francis. A gentle romance blooms, and Jon marries Ann. Back in London, Michael is now a member of Parliament and is anxious to advance a new social program called Fogartism. Fleur busies herself throwing fashionable parties and entertaining the visiting Francis Wilmot. At one such affair, society flapper Marjorie Ferrar’s catty remarks about her hostess are overheard by Soames, who causes a scene by ejecting her from the house. The incident snowballs into a libel threat against Fleur.

Marjorie Ferrar’s threats of libel action against Fleur are meant to extract money and a public apology, but Soames contrives to turn the tables and prove Marjorie guilty of immorality. American Francis Wilmot is torn between loyalty to Fleur and infatuation with Marjorie, who strings him along while secretly engaged to Sir Alexander McGowan, a hotheaded member of Parliament. Sir Alex heckles Michael’s maiden speech to the House introducing Fogartism, and a bathroom brawl follows. While Francis suffers with pneumonia, Fleur and Marjorie refuse to settle their differences.

Michael and Fleur drift further apart as last ditch efforts to settle the libel case fail, and Ferrar vs. Mont is heard in court. The episode is dominated by the bruising cross-examination of Marjorie by Fleur’s attorney, who reveals the plaintiff’s true views on morality, views secretly shared by the Monts. A settlement is reached, but at great cost: Fleur and Michael are ostracized, while Marjorie becomes the victimized belle of the ball.

Social fallout from the Ferrar case drives Fleur and Soames on a trip around the world. Michael joins them in Washington, DC, at the very moment when Irene, Jon, and Ann arrive. Soames narrowly averts a close encounter. Back in England, the General Strike of 1926 creates challenges and opportunities. Stainford, a down-and-out school chum of Val's, visits Winifred and pinches an antique snuffbox. Fleur runs a canteen for workers, as Jon and his family return to England for good. Michael seeks specifics about the family feud from June, while Fleur plans a chance meeting with Jon.

June lobbies for Soames to commission a portrait of Fleur. Michael buries himself in social work, but cannot mask his anxiety regarding his marriage. Likewise, Ann fears she is no match for Jon’s first true love, Fleur. Stainford appears at Val’s, asking for a hand-out, and is seen lurking around Val’s prized racehorse. Fleur subtly manipulates circumstances to continually run into Jon, who has trouble resisting her coy advances.

This moody episode illuminates Fleur’s restlessness, and Soames’s feelings of mortality. Looking toward the future, Soames installs young Butterfield in his law office to oversee Forsyte affairs, and alerts Fleur where he wishes to be buried. June’s current protege finishes a portrait of Ann, and begins separate pictures of Jon and Fleur. Holly asks Irene to settle with them, while Val deals with Stainford’s newest con, involving forgery.

The final episode of the series begins with an adulterous consummation, and ends in death. Jon succumbs to Fleur's seduction and immediately regrets his infidelity to his wife, Ann. It's apparent that Fleur will never give up, so Irene intercepts her at June's studio. Giving her a look at what the future will hold, she then drops a bombshell: Ann is pregnant with Jon's child. Soames advises a distraught Michael to stay the course, while Fleur must face the loss of Jon forever. A fire in Soames's picture gallery causes him a fatal injury, and as Fleur and Michael gingerly reconcile, Soames takes his final breath.