New mother Mary is finding it hard to come to terms with Matthew's shocking death. Robert and Violet fall out over how to handle the situation. Jimmy takes an interest in Ivy, while Carson receives an unwelcome reminder from his past.

Mary starts to get over the grief she is feeling, and takes an interest in the running of the estate. Lady Rose MacClare, Lady Flintshire's daughter, ends up in a predicament at a tea dance in York. Hughes attempts to convince Carson to heal an old wound.

A birthday party is planned for Robert, but Rose's surprise contribution to the event ruffles feathers above and below stairs. A new farming venture on the Downton estate comes with dramatic repercussions for Mary, and Edith receives more unwelcome news.

Mary's secret assignation is very nearly blown, but Violet's quick thinking saves the day. Robert is left red faced and furious when things do not go as planned. The events surrounding Green's death come under scrutiny once again. Mrs Patmore is devastated when old wounds are reopened but Carson is unsympathetic. Branson receives an interesting proposition concerning the future of the Downton estate. A figure from Violet's distant past suggests she has buried secrets of her own.

Simon Bricker returns to Downton to discuss the painting with Cora. Branson makes a difficult decision about Sarah Bunting. Rose encounters a handsome stranger. Determined to stop Isobel making the wrong decision about Lord Merton, Violet enlists the help of Doctor Clarkson. Charles Blake surprises Mary with a cunning plan to help her get exactly what she wants. Mrs Patmore seeks Carson's advice on a financial matter. Inspector Vyner returns with some ominous words of warning for Anna.

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.

Rosamund shows up at Downton following Edith's surprise departure. Rose is pleased to introduce Atticus and his parents to everyone. Anna and Bates think about a fresh start, now that that their problems seem to be behind them. Molesley is worried after Daisy becoming disillusioned with her studies. Mary and Blake seize a chance to send Gillingham a strong message.

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.

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.