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.

Winifred’s spendthrift husband Monty continues to rack up debts, while Soames has bigger marital problems. Attempting to improve his disastrous marriage, Soames engages Bosinney to build a mansion for Irene at Robin Hill, a country estate far removed from London. June suspects what Soames does not, that a powerful attraction exists between Irene and Bosinney. The attraction ignites a sexual liaison. Meanwhile, Old Jo surprises his son and Helene at their modest home, and attempts to mend the rift in the family. Young Jo astounds his relatives by attending the funeral of the eldest Forsyte, Aunt Ann.

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.

A thick fog envelops London, masking tragedy. Irene waits to run away with Bosinney, who is driven to distraction when hearing the news of Irene's rape. The next day, the Forsyte men gather en masse to support Soames, as the case of Forsyte vs. Bosinney is heard in court. The defendant is nowhere to be found, and he loses the case. June begs Old Jo to save Bosinney by purchasing the house at Robin Hill, now finished but empty. Brothers Old Jo and James, and their sons Young Jo and Soames, gather at the morgue to identify a man who was run down by a carriage in the fog; it is Bosinney.

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.

Queen Victoria’s isn’t the only death in this episode which also includes births, marriages, and divorce. Young Jolly dies in South Africa, while his sister Holly and Val Dartie marry and decide to remain there. Soames’s divorce is granted, uncontested, and he hurries to marry Annette and sire a son. His cousin Jo beats him to the punch when he marries Irene, who gives birth to their own son, Jon. On James’s deathbed, Soames tells a bold lie to allow his father to die in peace. Annette’s pregnancy is a difficult one; she can have no more children. Soames is introduced to the newest Forsyte, his one and only child, Fleur.

Eight more years pass, during which Helene dies and June, now financially independent, strikes out on her own. Monty continues his profligate ways, stealing Winifred’s pearls and sailing for Buenos Aires with a Brazilian dancer. On the advice of her brother Soames, Winifred begins divorce proceedings, regretting that the scandal may affect her son Val, now at Oxford. Young Jo’s daughter, Holly, meets Val, and an attraction is sparked. Meanwhile, Soames meets Annette, a young French woman who would welcome a wealthy husband. Determined to remarry and have a son, Soames seeks out Irene, looking for grounds for divorce.

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.

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.

Investigators shadowing Irene in Paris detect her growing affection for Jo, and gather enough evidence to proceed with Soames’s divorce. Val and Holly secretly become engaged, while Monty returns from Argentina, burnt out and broken. Against family advice, Winifred takes her wayward husband back. The Boer War galvanizes Uncle Timothy, and Val, Jolly, Holly, and June all head to South Africa to participate.

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.