The family and staff of Downton are shocked when they find that the heir to the title and fiancé of the Earl's daughter Mary perished on the Titanic, and the Earl hires a crippled army comrade as valet.

The new legal heirs to Downton, lawyer Matthew Crawley and his nurse mother Isobel are invited to live on the estate, but there is resentment of them as interlopers from both up and downstairs as interlopers.

Gwen's ambitions to leave service to become a secretary is discouraged, Bates takes painful steps to overcome his limp, and a dashing Turkish diplomat visiting Downton seems to work his charms on everyone - especially Mary.

Mrs. Hughes is reunited with an old beau, now a widower proposing marriage, and the Countess hires Matthew Crawley to see if the will can be broken and disagrees with his mother on a diagnosis.

After Bates' mother dies, he hopes his inheritance will buy him a divorce and allow him to marry Anna, Matthew announces his engagement to Lavinia Swire, and Sybil gets involved in the war effort.

April 1917. With John still absent, Isobel's butler Molesley makes a play for Anna but is rejected. Robert gets a new valet, shell-shocked ex-soldier Henry Lang, whilst William goes off to war. Edith learns to drive a tractor extremely well, and nearly succumbs to a kiss from the married farmer Mr. Drake. Sybil and Thomas work in the cottage hospital, where the latter begins to learn some humanity. At Isobel's suggestion - and to Violet's dismay - Downton Abbey is turned into a convalescent ward to ease the hospital's bed shortage. Mary invites middle-aged newspaper tycoon, and prospective beau, Sir Richard Carlisle to a dinner party, also attended by the Crawleys and Lavinia. Suspicious of Carlisle, Violet invites her daughter and sister to the Earl, Lady Rosamund, who is intrigued by the fact that Carlisle and Lavinia already seem well-acquainted. Carson, despite being taken ill whilst serving dinner, is still perceptive enough to suggest to Mary that Matthew is really the man for her.

Downton is turned into a convalescent hospital for the war wounded and through circumstances Thomas is given authority in its operation as Anna and Bates are briefly reunited and Branson plots against a heroic general dining at Downton.