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.