In 1927, approximately a year and a half following the conclusion of the acclaimed TV series, Buckingham Palace delivers exciting news to Robert Crawley and Cora Crawley, the esteemed Earl and Countess of Grantham: King George V (Simon Jones) and Queen Mary are set to visit Downton Abbey during their royal tour through Yorkshire. This development creates a stir, particularly for Violet Crawley, Robert's mother and the Dowager Countess of Grantham, who is unsettled by the inclusion of Maud Bagshaw, Queen Mary's lady-in-waiting, in the itinerary. Complications arise as Robert is Maud's direct cousin and both families grapple with inheritance issues surrounding Maud's estate. Violet strives to secure Robert's place in her eyes as a favored son, urging Maud to sign over her property to him.
As the royal entourage arrives, a series of staff members join Downton, including Mr. Wilson (David Haig), the Royal Page of the Backstairs, and others who exude arrogance towards the Downton staff, leading to tension among the servants. Meanwhile, Lady Mary Talbot, the eldest Crawley daughter, faces the burden of maintaining the estate's relevance in an ever-changing world. In her concerns that Thomas Barrow, the head butler, may not adequately manage the royal visit, she persuades the retired butler Charles Carson to temporarily return. Although Barrow reluctantly steps aside, his resentment simmers beneath the surface.
Among the problems occurring, the household's boiler fails, prompting a plumber's visit. Distractions breed jealousy as Daisy Mason (Sophie McShera), who is courting Andy Parker (Michael C. Fox), the resident butler, shows friendly interest towards the plumber. Subsequently, Andy, consumed by jealousy, sabotages the boiler's repair, much to the dismay of Monsieur Courbet, the royal chef. The following day, as the weather clears, the King and Queen's presence creates palpable excitement and challenges, including setting up for their reception while contending with unexpected rain.
The royal family's arrival is marked by an awkward exchange between Violet and Maud as tensions between the two families resurface. Amidst this, Major Chetwode (Stephen Campbell Moore) arrives in Downton Village, misidentified by Tom Branson, who is the Crawleys' Irish son-in-law. In a dramatic confrontation, Tom thwarts an assassination attempt on the King, revealing not just his valor but also complicating loyalties.
As the narrative unfolds, romantic entanglements blossom amidst the chaos; Tom finds a connection with Lucy Smith (Tuppence Middleton), Lady Maud's maid. With storms brewing both outside and within, unexpected deliveries, such as 200 chairs for a parade, only heighten the day’s urgency.
In a twist, Violet learns that Maud's intended heir is not Robert, but rather Lucy, her illegitimate daughter. This vitally ensures the reconciliation of the two families, while the evening culminates in a royal ball at Harewood House. During this grand event, Princess Mary expresses her gratitude towards Tom for saving her marriage, while the King acknowledges Tom's heroic act.
Through a series of events, misunderstandings are clarified, and friendships solidified. Maud confesses her secret to Violet, leading to a heartwarming reconciliation. Ultimately, Lady Mary, grappling with her grandmother's declining health, finds comfort knowing Downton’s legacy is securely in her hands, while Tom and Lucy share a moment of solace in the shadows, dancing and lost in the melodies of the ball.
