Relations between Robert and Cora continue to be strained. Blake's scheming starts to come to fruition when Mary is unexpectedly pitted against her love rival. Edith receives some terrible news and decides to take drastic action. Following a mysterious tip off, Baxter becomes embroiled in the investigation of Green's death. Thomas' condition deteriorates, prompting him to finally reveal the truth to Baxter. Love is in the air for more than one member of the Crawley family.

Preparations are under way for Rose's wedding, but family tensions on both sides threaten to derail proceedings. Edith's arrangement with Marigold is happily working out, but how long can she and Cora conceal the truth from Robert? Denker reveals an unexpected side to her personality, which causes trouble for more than one of the servants. With the war memorial due to be unveiled, Mrs Patmore is still upset, but Robert has something up his sleeve. Branson comes to a heart-wrenching decision, and the storm clouds that have been gathering over Anna and Bates finally burst.

It's grouse shooting season and Rose’s father-in-law, Lord Sinderby, has rented out Brancaster Castle in Northumberland and invited the Crawley family to a shooting party. While good sport is enjoyed, Stowell has an axe to grind and a scandalous secret threatens to undermine the holiday. Surprises are in store as the families become better acquainted with each other and some new faces arrive on the scene.

It is now 1925, when secrets and rifts threaten the unity of the family, while those below stairs are faced with social changes.

Preparations for the wedding of Mr Carson and Mrs Hughes are under way. Edith meets up with an old acquaintance. Spratt has something to hide.

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 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.

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.