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.

Bates sees Thomas stealing a bottle of wine, the footman tries to frame him for petty theft, and Mrs. Crawley tries to convince the Dowager Countess to surrender her annual entitlement prize for best bloom in the village.

O'Brien and Thomas continue their efforts to frame Bates, Mary falls in love with Matthew, and Sybil is injured in a political disturbance and Branson is blamed.

Mary finds it was Edith who let the ambassador know about Pamuk's death, Cora announces her pregnancy, and Thomas and O'Brien hatch new slanders against Bates as War clouds loom.

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.

Matthew and William are missing in France, the mounting tensions between Cora and Isobel result in Isobel's volunteering for France, Bates returns to Downton, and the servants open a soup kitchen for unemployed veterans.

August 1918: Deserted by the baby's father, Ethel has been installed with her child in a cottage on the estate by a sympathetic Mrs. Hughes, who brings her food. Jane, a war widow, has taken Ethel's place as a maid. Vera Bates returns, having spent John's money but with no intention of sticking to their arrangement and planning to make money by selling the story of Lady Mary's indiscretion to Carlisle's newspaper. Tipped off by Anna, Mary goes to see him and explains all, thus scotching Vera's plan as the newspaper owner threatens her with libel action. Having returned to the war, both Matthew and William sustain severe injuries at the Battle of Amiens, fatally in the case of William, who marries Daisy. Mary is glad to become a hospital auxiliary to nurse Matthew, though his spinal damage will, in all probability, leave him paralysed from the waist down and impotent.

An unrecognizable burn victim turns up at the convalescent home claiming to be Mary's presumed drowned cousin Patrick, Carlisle plans to lure Carson from Downton, and Bates' wife reneges on her divorce agreement.

Ethel confronts her baby's grandparents, Bates thinks he might have played a role in his wife's death, Branson and Sybil elope, and Carlyle tries to recruit Anna to spy on Mary.

The Spanish Flu disrupts plans for Matthew's wedding when Cora, Lavinia, Carson, Molesley, and other staff members fall ill. Ethel has to decide the future for her son, and Jane resigns from Downton.

Christmas 1919. Downton Abbey is hosting a lavish Christmas party, yet despite being the season of goodwill, tensions are rife and Bates' arrest has cast a shadow over the festivities.

Spring of 1920. Wedding guests descend on Downton Abbey, where disasters threaten. One of which is Cora's freewheeling American mother, who tries to loosen up her in-laws.

A conflict between two members of the house staff escalates. Meanwhile, the family tries to impress Martha by planning a lavish dinner party.

The fate of Downton Abbey hinges on a letter from a dead man. Edith and Sir Anthony face their own fateful moment. Mrs. Hughes confronts a crisis.

August 1920: Two social revolutions arrive at Downton Abbey: the Irish civil war and the fight for women's suffrage. A mysterious conspiracy keeps Anna and Bates apart.

The Crawleys excitedly await as the new addition to the family is soon to come. Meanwhile, Anna attempts to uncover evidence for Bates's innocence. But then, when tragedy strikes, will things ever be the same again?

Things go badly amiss at Downton Abbey. Robert and Cora are not speaking. The servants are shunning Matthew's mother, Isobel. And Matthew and Robert have fallen out. Bates takes a gamble.

A vindictive Ms. O'Brien hatches a damaging plot. Edith makes a major decision. Matthew devises a major plan for changing things, and Tom's brother pays a visit.

Change arrives in a big way for several key characters at Downton Abbey. A yearly cricket match with the village sees old scores settled and new plots are hatched.

The Crawleys go to Duneagle Castle to celebrate Christmas with the MacClares. Everyone is delighted as they fish, hunt, and dine. Unfortunately, when the Crawleys return, tragedy strikes. Will things ever be the same again?

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.

Ethel confronts her baby's grandparents, Bates thinks he might have played a role in his wife's death, Branson and Sybil elope, and Carlyle tries to recruit Anna to spy on Mary.

Change arrives in a big way for several key characters at Downton Abbey. A yearly cricket match with the village sees old scores settled and new plots are hatched.

A vindictive Ms. O'Brien hatches a damaging plot. Edith makes a major decision. Matthew devises a major plan for changing things, and Tom's brother pays a visit.

The Crawleys excitedly await as the new addition to the family is soon to come. Meanwhile, Anna attempts to uncover evidence for Bates's innocence. But then, when tragedy strikes, will things ever be the same again?

The fate of Downton Abbey hinges on a letter from a dead man. Edith and Sir Anthony face their own fateful moment. Mrs. Hughes confronts a crisis.

A conflict between two members of the house staff escalates. Meanwhile, the family tries to impress Martha by planning a lavish dinner party.

Spring of 1920. Wedding guests descend on Downton Abbey, where disasters threaten. One of which is Cora's freewheeling American mother, who tries to loosen up her in-laws.