The elder generation of Forsytes gather at Uncle Timothy's to exchange gossip about two of the younger members of the family: James's daughter Winifred is engaged to the charming but financially questionable Montague Dartie, while Jolyon's married son, Jo, commits adultery with the Austrian governess, Helene. Old Jo overhears the gossip at Winifred's wedding, while Helene has bittersweet news for Young Jo: they are going to have a child.

Young Jo’s decision to divorce his wife to live with Helene and their child shocks the family, and causes Old Jo to adjust his will in favor of his granddaughter June. Elsewhere, stuffy lawyer Soames Forsyte is enchanted with young but penniless Irene Herrin, whose ethereal beauty will be the catalyst for much of the series.

Two sons are born. Winifred and her profligate husband Monty must appeal to her father James to ease Monty’s debts, as Winifred gives birth to her son, Val. The accidental death of Young Jo’s first wife allows Jo and Helene to legally marry. Helene gives birth to the third generation of Jolyon Forsytes, nicknamed Jolly, while Old Jo assumes parental authority over granddaughter June. Soames methodically and relentlessly pursues an unwilling Irene, who finally accepts his marriage proposal, but with one condition: that he release her from the marriage if they are not happy. Their wedding night sets the stage for despair ahead.

Several years have passed, and the family is abuzz over the engagement of young June Forsyte to bohemian architect Phillip Bosinney. June’s best friend and confidante, Irene, endures an icy marriage to Soames, but sexual sparks fly when she and Bosinney meet. In the poorer part of town, Helene’s extreme guilt over keeping Jo from his daughter June is making her ill, while the estranged Old and Young Jo take tentative steps toward reconciliation.

As the affair between Irene and Bosinney becomes more and more public, Soames threatens legal action against the architect regarding cost overruns at Robin Hill. June hides her confusion with stubborn allegiance to Bosinney, while, enraged and frustrated, Soames violently asserts his marital rights.

Distraught, destitute, and alone, Irene returns to Soames for one last night, then deserts him for good. Four years later, Jolyon and his family have settled into the house at Robin Hill. While the others are in Spain, Old Jo comes upon a melancholy Irene, sitting in the copse and remembering her love for Bosinney. A gentle friendship blooms between the old man and the fallen woman. Jolyon dies peacefully, but not before leaving a surprise in his will: a bequest for Irene.

The South African Boer War creates anxiety for James, while Winifred and Soames attempt to extricate themselves from their marriages. Soames hires a private investigator to follow Irene, in hopes of gathering evidence for a divorce. Irene flees to Paris, where Jo joins her and begins to fall in love. Back home, Val and Holly continue their romance, despite Holly’s brother Jolly, who despises Val and challenges him to join him to fight in the Boer War. Soon Jo is summoned back to Robin Hill: Jolly is headed to South Africa.

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.