Mary's past catches up with her when an unwelcome visitor delivers an ultimatum, as secrets and rifts threaten the unity of the family. News about the running of Downton Hospital puts Violet on the warpath, while those below stairs find social changes are putting their futures in jeopardy and Daisy's good intentions at the Mallerton House auction have disastrous consequences.

Plans for the wedding are gathering steam, but Lady Mary's meddling lands Carson in trouble with Mrs Hughes. Cora and Violet clash over plans for the hospital takeover, and Thomas's job search is proving disheartening. Mary comes up with a fresh idea that offers Anna fresh hope, prompting a trip to London, and Lady Edith faces challenges at the magazine although it's a problem closer to home that reaches breaking point. A day for the family and servants ends in panic and leaves Robert facing a difficult decision.

Mr Carson and Mrs Hughes' wedding is fast approaching, but a misunderstanding threatens to derail the occasion, and when the big day arrives it also brings a few unexpected guests. Meanwhile, Edith reconnects with an old acquaintance, who comes to her aid in her hour of need, and over at the Dower House, Spratt has something to hide, meaning Denker is hot on his trail. Daisy jumps to conclusions over one of the countess's plots, and tensions mount over the hospital takeover, prompting Isobel to offend a friend.

Tom and Sybbie return to Downton, but the peace is disrupted by an unexpected visit from an old friend. Baxter is faced with a dilemma after an encounter with Sergeant Willis, while Thomas struggles to fill Carson's shoes. Anna goes to London with Mary to deal with a crisis, while her secrets seem likely to be exposed. Daisy is not happy with Cora, and Mary has an admirer.

Neville Chamberlain visits Downton for dinner, and is drawn into Violet's plan to save the hospital, with distressing consequences for everyone. Andy finds an unexpected confidante while helping Mr Mason move into Yew Tree Farm, Mary is uncertain if she has met her match, Edith meets Bertie Pelham in London, and Spratt comes to Denker's aid. Mrs Hughes struggles to keep Carson happy, while Mary thinks secrets are being kept from her.

The house is made open to the public to raise money for the hospital, an act that divides opinions among the family and the servants. Tensions between Violet and Cora come to a head as a decision is made about the hospital takeover, Mr Dawes has a proposition for Molesley, and Baxter is thrown into turmoil by an unexpected letter. Carson is not happy with Thomas, while Mary and Edith hope to overcome their past and find new happiness with the men in their lives.

The Crawleys watch Henry compete in a motor racing event, an occasion that leaves Mary reeling. Violet hatches a plan which surprises the family and delights Miss Denker, and learns more about Amelia's intentions in befriending Isobel. Molesley and Daisy rise to the challenges of examination day, Andy's secret is brought to light, Thomas feels isolated, and Mrs Patmore and Mrs Hughes teach Carson a lesson.

Bertie's circumstances change suddenly, leaving Edith uncertain whether to risk telling him the truth or keep her past a secret at all costs. Mary wonders if she can ever make peace with her sister as hostilities escalate, Spratt shows some hidden talents and Molesley's first day as a teacher does not go according to plan. Mrs Patmore's bed and breakfast attracts unwanted attention, leaving her in need of the Crawleys' help.

Mary endeavours to build bridges with her sister while Edith’s secret continues to pose a threat, despite having nothing left to lose. As Henry settles into the role of husband and stepfather, finding his place at Downton proves more difficult. Below stairs, Carson faces some personal challenges, which prove that even he is not invulnerable to change. The last ever episode offers all the love and loss, happiness and heartbreak of Downton Abbey, and as the family and servants prepare to welcome 1926, they celebrate an unforgettable New Year’s Eve together at the great house.