Set in 1922, the fourth series sees Lady Mary mourning her loss as the Crawley family and the servants who work for them remain inseparably interlinked.
Mary is finding it hard to come to terms with Matthew's 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.
An extravagant house party at the Abbey gives the Crawleys a chance to reconnect with old friends. But some guests prove more welcome than others, and along with the celebrations come skulduggery and heartache.
Amid the fallout from the eventful house party, Mary faces a pressing question about her future. Carson reveals surprisingly intimate details of his former life, and a trip to the bright lights of London sees Rose put the reputation of the Crawleys in jeopardy.
Change is in the air as an exciting opportunity for Alfred creates tension between Ivy and Daisy. Isobel appeals to Violet's charitable nature, but it does not come without a fight, and Branson considers a new future for himself and baby Sybbie.
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.
Cora's playboy brother has got himself into a spot of bother and Robert has to make a trip to America to bail him out. Blake and Mary are forced to put aside their differences to deal with a new farming venture on the estate. When Violet is taken ill, help comes from an unlikely quarter. In spite of Mrs Hughes, Mrs Patmore and Carson's attempts to keep the peace, romantic tensions below stairs cause problems.
Rosamund comes up with a plan to help Edith deal with her problem, but Violet may be about to see right through it. Isobel is the object of someone's attention, while Mary asks Gillingham for a favour. Rose is frustrated when things do not go her way and Alfred realises too late that he may have made a mistake. Just as Bates and Anna seem to be getting over the worst, some unexpected news shakes her up.