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 party offers the Crawley family the chance to reunite with some old friends. Some of the guests though prove more welcome than others. Dame Nellie Melba performs.
Following the Abbey's eventful party, Mary needs to answer an important question about her future. Carson reveals some intimate details about his former life whilst Rose's trip to London sees her at risk of jeopardising the Crawley family's reputation.
The are changes planned at the Abbey as an exciting opportunity for Alfred causestension between Ivy and Daisy. Branson thinks about a new future for him 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 brother Harold has managed to get himself into some trouble and Robert needs to travel to America to bail him out. Blake and Mary have to try and put aside their differences to help with the new farming venture on the estate.
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.