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.
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.
With Robert's birthday just around the corner party plans are under way. Rose's surprise contribution to the event ruffles feathers in the household. Mary needs to deal with some dramatic repercussions after a new farming venture is launched on the Downton estate. Edith gets some 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 wy to try and help Edith deal with her problem. Rose is not happy after things fail go her way. Alfred realises too late he could have made a mistake. Anna receives some unexpected news as she and Bates look lile they are getting back on track. Isobel becopmes the object of somebody's attention.
The family and servants make their way to London for the summer season. The family's London residence, Grantham House, is seen for the first time. Martha Levinson and Cora's brother Harold make an appearance.