Plot
In 1927, the Crawleys receive word that King George V and Queen Mary intend to visit Downton during their royal tour of Yorkshire, exciting both the family and the staff. As the servants begin preparations, Lady Mary feels that house butler Barrow is ill-equipped to manage such an important event and thus recruits former butler Carson to briefly exit retirement in order to assist, much to Barrow's chagrin. When the boiler breaks down, a repairman shows up and seems to be flirting with the assistant cook Daisy, making the footman Andy jealous. Shortly thereafter, members of the royal staff begin to arrive at Downton in advance of the King and Queen themselves. While Richard Ellis, the King's valet, treats the Downton staff kindly, the rest of the entourage are arrogant and rude, making it clear that the royal staff intends to supplant that of Downton for the duration of the visit. In addition, small items keep disappearing from the house.
Bertie and Edith Pelham, Marquess and Marchioness of Hexham, arrive the day before the royal couple is due to visit. Later that evening, chairs for the parade seating are delivered during heavy rain and Lady Mary leads a messy group effort to position the chairs for the next day. The weather clears and the King and Queen arrive at Downton, where they are introduced to the Crawley family. Violet exchanges cold pleasantries with the queen's lady-in-waiting Maud Bagshaw, who is Robert's first cousin once removed, and Tom meets her lady's maid Lucy Smith. Violet begrudges Maud for her decision to bequeath her estate to Lucy rather than to Robert.
Meanwhile, in the village, Tom encounters a mysterious man identifying himself as Major Chetwode, whom Tom initially believes is assessing security ahead of the royal visit and parade. Later, when the parade is set to begin, Chetwode prepares to assassinate the King in the staging area but is thwarted by Tom, whom Chetwode mistakenly believed was an ally because of their shared Irish origins, and Lady Mary, who disarms him after Tom pins him to the ground. After the parade, Tom encounters a woman sobbing on Downton's lawn. Unbeknownst to him, the woman is Princess Mary. She laments her failing marriage, but Tom lifts her spirits and ultimately motivates her to remain with her husband.
Anna and Mr Bates rally the Downton staff into retaking control of Downton's operations while the royal couple is still visiting. Barrow and Ellis trick Mr Wilson, the Page of the Backstairs, into ordering some of the royal staff to return home to London early. Anna slips a strong sedative into the tea of the royal chef, Monsieur Courbet, and Wilson himself is "accidentally" locked in his room. Anna has surmised that Miss Lawton, the Queen's dresser, is stealing the house's items and leverages her knowledge of Lawton's crimes to compel her to alter a gown for Lady Hexham. Then the Downton staff, with the assistance of the footmen Mr Molesley and Albert, manage the dinner that marks the end of the royal visit. The revised menu elicits high praise from the King, which in turn prompts Molesley to break protocol and announce that it was the Downton staff, not the royal staff, that produced it.
That evening, Barrow and Ellis make a trip to York. Ellis visits his parents, while Barrow waits for him at a pub. A man at the pub invites Barrow to an underground gay nightclub, but shortly after their arrival there, police raid the venue and arrest the attendees. Ellis, also a closeted gay man, soon learns what happened and uses his position in the royal household to get Barrow released from police custody. Isobel deduces that Lucy is Maud's illegitimate daughter, which is why she stands to inherit Maud's estate. Isobel challenges Maud to explain the situation to Violet.
The next morning, Mary's husband Henry Talbot returns home after travelling abroad and joins the Crawleys at Harewood House, along with the royal family. During the ball that evening at Harewood, Princess Mary informs her parents that Tom helped save her marriage by convincing her to remain with her husband and she encourages the King to speak with Tom. He approaches Tom and expresses gratitude, also implying that he's aware of Tom's role in thwarting the assassination attempt at the parade.
The King releases Bertie from his obligation to join the Prince of Wales on a three-month tour of British Africa, which the King had requested earlier in the visit. Bertie had initially accepted the commitment, but he later wished to back out of the trip after learning of Edith's pregnancy. Thanks to Cora and the Queen's intervention, the King had a change of heart and rescinded his request. Cora reveals "Bertie asked the King...I asked, the Queen". Meanwhile, Maud privately confesses to Violet that Lucy is her daughter, which is why she made Lucy her heir instead of Robert. Violet finally understands the situation and the two reconcile. Violet then begins plotting to keep Maud's estate in the Crawley family in the future through a union of Tom and Lucy, as she had taken notice of the pair's budding romance earlier.
Finally, Lady Mary quietly asks Violet about her visit to London. Violet informs her that she had gone for a doctor's visit, during which she learned that she may not have long to live. Mary is distraught, but Violet assures her that Downton and its legacy are safe in Mary's hands. As this conversation continues, Tom finds Lucy on the terrace and the two dance in the dark while listening to the music played inside at the ball.
