When the Titanic goes down, Lord Grantham loses his immediate heirs and his daughter Mary loses her fiance, throwing Downton Abbey and its servants into turmoil. The new heir turns out to be Matthew, a lowly lawyer with novel ideas about country life.
Mary entertains three suitors, including a Turkish diplomat whose boldness leads to a surprising event. Downstairs, the shocking former life of Carson, the butler, is unmasked and Bates risks his health to remain valet.
Growing into his role as heir, Matthew brings out the bitter rivalry between sisters Mary and Edith. Servants Thomas and O’Brien scheme against Bates, while head housemaid Anna is increasingly attracted to him. Lady Violet’s winning streak in the flower show is threatened.
The heir crisis at Downton Abbey takes an unexpected turn. Meanwhile, rumors fly about Mary’s virtue. Her sister Sybil takes a risk in her secret political life. Anna unearths Bates’ mysterious past and O’Brien and Thomas plot their exit strategy.
As the village prepares for the annual flower show, Isobel finds herself in conflict with Violet. Mary is introduced to a potential suitor, but she is taking notice of Matthew.
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.
Cora makes a discovery that could throw the inheritance issue into chaos. Under pressure from her aunt, Lady Rosamund Painswick, Mary keeps Matthew waiting for an answer to his proposal.