Rosamund arrives at Downton following Edith's shocking departure; Edith's secret may be exposed to the family; Rose is delighted to introduce Atticus and his parents to Downton for the first time.
Rose's father-in-law, Lord Sinderby, has rented Brancaster Castle in Northumberland and invited the Crawleys to a shooting party.
Preparations are under way for Rose's big day, but family tensions on both sides threaten to derail proceedings; Edith's arrangement with Marigold is happily working out, but she and Cora continue to conceal the truth from Robert.
Relations between Robert and Cora are strained; Blake's scheming starts to come to fruition; Mary is unexpectedly pitted against her love rival.