The film centers on Lady Mary’s private and public separation from Henry Talbot, finalizing a formal divorce that unleashes a scandal across London high society. Shunned by metropolitan society, Mary retreats to Yorkshire with her father, Robert, who rallies the household to shield her.
Meanwhile, the ripples of the 1929 stock market crash arrive in England. Cora's brother, Harold Levinson (Paul Giamatti), arrives from the United States with business updates, forcing Robert to confront the grim financial costs of keeping Downton running in its current form. Rather than resorting to outside bailouts, Robert formally acknowledges that his traditional management style is obsolete.
In an emotional council, Robert and Cora make the executive decision to step down from governing the estate, arranging to relocate their living quarters and leaving the primary management and master keys entirely to Mary and her children, with young George recognized as the eventual master of the new decade. Carson, in parallel, officially hands all duties to the younger downstairs leadership.
The climax converges at the grand Yorkshire Show, where Mary stands tall before the county, brushing aside social snobbery. The film closes on an expansive, sun-drenched montage of the household gathered in harmony: Thomas Barrow happily visiting from London, the staff sharing toasts downstairs, and Mary standing in the great hall before Violet’s portrait, ready to lead Downton Abbey into the modern world.