The lives of the Crawley family and Downton Abbey's servants are changed for ever when the sinking of the Titanic leaves the estate without its heir and his son. The arrival of a new valet shocks the staff, who scheme to make his stay as short-lived as possible.

Everyone is anxious at the arrival of Matthew Crawley, the heir presumptive, and his mother, Isobel. Matthew encounters problems settling in to this new life whilst Isobel and Violet are immediately at daggers drawn. Carson is mortified to be confronted with his past and Gwen seems to have a secret correspondent.

Cora's attempts at matchmaking are dashed when Mary is smitten not by intended target Evelyn Napier, but by his handsome friend, Turkish attache Kemal Pamuk. The couple's mutual attraction sparks a train of events whose consequences ripple through the entire household. Gwen is deeply humiliated as her secret ambition is exposed, and Bates takes extreme action to cure his limp.

The fair arrives in the village and Mrs. Hughes finds herself the centre of speculation when she meets a former suitor who makes her question her position at Downton. Meanwhile, Violet and Isobel lock horns over a medical ailment affecting Molesley, and Carson fears there is a thief at work in the grounds.

As the village prepares for the annual flower show, Isobel finds herself in conflict with Violet, and Mary is introduced to a potential suitor, but it would seem she only has eyes for Matthew. Anna realizes who is to blame when a valuable snuff box goes missing, and Mrs. Patmore tearfully reveals her fears to Carson that her career may be coming to an end. Meanwhile, O'Brien engineers a way for Edith to take revenge on Mary.

Sybil's determination to pursue her political leanings ends up causing her an injury when a fight breaks out at an election campaign meeting. Sybil's sisters note Matthew's tenderness in caring for her.