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.

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.

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.

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.

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.

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.