Change is in the air as the politically awakened Sybil rallies for the women's vote, in direct violation of her father's rules. But when Sybil is swept up in the violence surrounding the reading of the election results, Matthew wins a heart by defending the girl and bringing her to safety. Meanwhile, back at Downton Abbey, persistent rumors about a family member cause a rift between Cora and Violet. And, a surprise announcement from Cora complicates the larger issue of Downton's fate.
In London, Mary gains a shocking insight, and her Aunt Rosamund — displaying a more than passing resemblance to her mother, Violet — freely dispenses dubious advice to Mary. Meanwhile, Thomas and O'Brien enlist the reluctant, smitten Daisy to bring about Bates' downfall, but Bates seems determined to do it himself, much to the distress of Anna, who finds an opportunity to delve into Bates' past. Concerned about the security of her position, O'Brien sets her spite on a new target, and a misunderstanding provokes a dangerous act of sabotage. Again, Downton is to be hobbled by the entail.
At a resplendent garden party, actions and betrayals come home to roost, and important news arrives that dwarfs the issue of inheritance.
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
The women's vote is up for consideration, and Lady Sybil, politically awoken, attends a rally with the socialist chauffeur, Branson. Bates accidentally reveals her attendance to Robert, who is furious, castigating her and allowing Branson to keep his job only by the skin of his teeth. Thomas blames Bates, who counters with a veiled threat to expose Thomas as a thief. But when Daisy gushes to Thomas that she'd do anything for him, he considers her an ally in his and O'Brien's plan to save his own skin by casting suspicion on Bates as the thief of the missing wine. Meanwhile, Bates confides to Anna that he would never actually turn Thomas in, as he wouldn't want to be the cause of someone losing his job.
The London season is set to begin, a concern for Cora who insists that Mary's situation absolutely must be settled. Carson has shared with Cora a letter from a valet friend whose employer has heard scurrilous stories impugning Lady Mary's virtue. Violet too has been privy to such gossip — she questions Cora to learn that the rumors are indeed all true. Appalled, she blames Cora for her part, but Cora stands by Mary.
Only Edith's lot is improving; when Mary rejects an automobile ride with Sir Anthony Strallan, Edith volunteers her company and the two have a genuinely enjoyable time. When he next calls, it is for Edith, who blushes, pleased.
Thomas and O'Brien have struck against Bates, lying to Carson that they've witnessed the valet handling the cellar key and wine, even enlisting Daisy to lie along with them. While Bates denies the charges, he won't reveal Thomas's theft, and Carson is left pondering how to respond to the charges. Anna assures Bates that she will tell the world the truth about him.
In conversation, William shares a personal detail about his family with Daisy: there are no lies in his house. This sets the impressionable kitchen maid thinking and looking at William in a new light. She confesses to Carson that she lied about Mr. Bates and the wine.
Sybil receives Robert's permission to return to Rippon with Branson in order to attend a charity meeting. But she has lied to both her father and the chauffeur, leaping out of the car upon arrival to attend the much anticipated election results. Branson protests, but the rally quickly becomes a mob when liberal party activists arrive to fight the Tories. Matthew, leaving work, is drawn into the assembly square, spots Sybil, and tries to get her to leave.
Amidst the potential violence, Branson tries to reason with a ruffian but is shoved away. The man takes a swing at Matthew, who tries to defend them, and Sybil gets shoved, hitting her head as she falls. Panicked, Branson carries the unconscious girl to the car and takes her to Crawley House. There, Isobel attends to Sybil while Branson fetches Mary. Sybil will be fine but must face her father. Mary defends Branson to her enraged father, who is ready to fire the chauffeur. Only Sybil's threat to leave if Branson is dismissed saves the young man's job.
Carson has assembled O'Brien, Thomas, Daisy, and Bates, as well as Anna and Mrs. Hughes to clear up the matter of the wine theft. While O'Brien sticks to her lies and Thomas tries to weasel out of his accusation, Bates still won't point the finger at Thomas. In fact, the exonerated man asks that Carson, Anna, and Mrs. Hughes remain to hear a new confession: that while he did not steal wine, he actually was, until two years prior, a drunkard who had been imprisoned as a thief. He requests that he be dismissed, but, reluctant to take Bates as a reprobate, Carson will consider the resignation and speak to Lord Grantham about the matter. Neither will Anna so easily dismiss Bates; she tells him there isn't a better man than he and the two almost kiss, but are interrupted.
Over sandwiches, alone after the drama of Sybil's injury, Mary and Matthew talk plainly and even kiss. Mary goes in to wish her mother goodnight and declares that Matthew has proposed to her. She loves him — perhaps has for a long time — but feels that she has to tell him about the Pamuk incident before accepting his offer.
The family leaves Downton Abbey for the London season. Mary has told Matthew she will give him her answer when they return.
Yet upon their return, nothing is resolved. The future is uncertain after the assassination of the archduke, war could be imminent, and rumors about Mary have spread throughout London society. Adding to the uncertainty, Cora and Robert receive the joyous yet thoroughly unexpected news that Cora is pregnant. The entail is again at play, and this time, should a son be born, the one pushed aside would be Matthew. Robert assures Matthew that should that happen, he would be provided for, but the young man is on tenterhooks awaiting Mary's answer and cannot envision a future at Downton without her.
Staying on in London, Mary receives bad advice from her Aunt Rosamond: wait to give Matthew an answer until the sex of the baby is learned and the issue of inheritance is resolved. Mary also receives a visit from Evelyn Napier, who tells her that the source of the gossip about her and Kemal Pamuk is none other than her sister Edith. Yet Mary is reluctant to give Matthew an answer until she is sure that she wants to marry him. When she returns, Matthew is hurt and growing weary of waiting.
Robert arranges for the nearly blind cook, Mrs. Patmore, to see a specialist for surgery in London. Anna will accompany her there while Mrs. Bird, the cook for the Crawleys, will temporarily fill in at Downton. While in London, Anna visits the army barracks and then Bates's mother, on a mission to get to the bottom of Bates's confession. There, she learns that after the war and his injury, he drank too much, guilty that he had ruined his wife's life. But it was she who stole — he took the blame for her. Anna returns to London with Mrs. Patmore, who can now see, and tells Carson the truth about Bates.
O'Brien overhears part of a conversation between Cora and Robert and takes it to mean that they are planning to fire her. She is further convinced when Thomas overhears Violet asking Cora about the advertisement she'd placed for a new lady's maid. The advertisement is actually for Violet, whose maid is leaving to marry, but Thomas doesn't know this and when he tells O'Brien, she is enraged. Other references to a new lady's maid drive O'Brien to distraction, and when Cora is in her bath and a bar of soap breaks on the floor, O'Brien intentionally leaves a piece for Cora to slip on. She immediately feels regret for what she has done and tries to stop Cora from getting out of the tub, but it is too late. Cora falls and loses the baby who, they learn, was a boy.
Molesley has caught Thomas stealing from Carson's wallet, and reported it to Carson, who will fire him after the garden party. But Thomas already has other plans — with war potentially looming, he has asked the doctor to help him secure a place in the army medical corps. Confident that he is on his way out, he unleashes extra cruelty among his coworkers and William finally stands up to him with a powerful punch. Gwen, too, is on her way out. Sybil had put her in touch with Mr. Bromidge, the man who is installing telephones in Downton Abbey, and she appears to be a perfect fit for the secretary he seeks.
At the garden party, Gwen learns from an overjoyed Sybil and her love struck cohort Branson that she has indeed gotten the job. Thomas, too, learns of his new position in the army when the doctor gives him his papers. Daisy apologizes to William for being so cruel to him, and O'Brien guiltily tends to Cora. She is horrified to learn that from Violet that it was she seeking a new lady's maid. Mary deftly sabotages Edith's future with Sir Anthony, who had intended to propose but instead retreats, humiliated. For Mary, who pointedly raises her glass to a heartbroken Edith, revenge is sweet, but not for long. A conversation with Matthew reveals that it is over between them as Matthew, who is so bitterly hurt at her delay, feels he must leave Downton and start over. She sobs, comforted by Carson.
A telegram arrives and Robert addresses his family and their guests. He grimly announces, "I regret to inform you that we are at war with Germany."