With Mary and Matthew’s wedding imminent, financial ruin threatens when it develops that Robert has sunk most of Cora’s fortune in a railway company that has gone bankrupt. The family now faces the loss of Downton Abbey and the radical downsizing of their way of life.
At the same time, Matthew learns that the father of his late fiance, Lavinia, has died, leaving him a huge fortune. Since Matthew is heir to Downton Abbey, this windfall looks like the miraculous salvation of the estate. However, Matthew feels honor-bound to refuse the unexpected legacy, since he is convinced that he broke Lavinia’s heart, leading to her death.
Events reach a crisis point on the eve of the nuptials, when Mary accuses Matthew of disloyalty to the family over his act of principle. “Deep down, you're not on our side!” she cries. Branson, who is Matthew’s best man and who has come from Ireland with Sybil thanks to money secretly wired by Violet, talks Matthew into making amends with Mary. The wedding goes ahead, but with Matthew still adamant about refusing the inheritance.
Adding to the excitement is the arrival of Cora’s millionaire mother, Martha, from America. When an elaborate dinner party faces disaster after a kitchen mishap, Martha organizes an impromptu indoor picnic and musical soirée. Her delight at upending Downton’s rigid protocol makes it difficult for Mary and Violet to pursue their plan to woo Martha into using her fortune to save the estate. Anyway, explains Martha, her money is tied up.
Meanwhile, Edith, the only unmarried sister, coaxes Sir Anthony Strallan ever closer to proposing. Sir Anthony realizes he is too old for Edith—not to mention crippled in the war. But Edith defeats all attempts to end the relationship and winds up engaged to the affable widower.
Downstairs, the staff is shorthanded owing to Robert’s sudden compulsion to economize. One result is that Daisy goes on strike. Furthermore, in a moment of inattention, Robert agrees to O’Brien’s scheme to get her nephew Alfred hired as a footman. This creates a rival for Thomas, and drives a wedge between Thomas and his soul mate in conspiracy, O’Brien.
In prison, Bates shows his dark side when he roughs up his cellmate, who has taunted him about being a murderer. On the outside, Anna is desperately searching for a clue that will prove Bates’s innocence. Also confronting the seamy element is Isobel, whose latest cause is helping fallen women. One supplicant who approaches her is Ethel, the Downton maid who was dismissed and later turned up with a baby.
Worst of all, Mrs. Hughes fears she has breast cancer and waits to hear the test results from Dr. Clarkson. Mrs. Patmore attempts to keep her spirits up, but the cook’s forthright bedside manner has quite the opposite effect.