Rose's father-in-law, Lord Sinderby, has rented Brancaster Castle in Northumberland and invited the Crawleys to a shooting party.
Preparations have begun for Rose's big day, but there are tensions on both sides of the family that threaten to derail proceedings.
Rosamund arrives at Downton following Edith's shock departure and a secret may be revealed; Rose is delighted to introduce Atticus and his parents to Downton for the first time.
Robert and Cora's relationship is strained; Blake's scheming starts to come to fruition; Mary is unexpectedly pitted against her love rival.
The team are in imminent danger when they expose a web of lies and deceit.