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.

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.

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.

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.

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.

An old acquaintance comes to Edith's aid, while Mr Carson and Mrs Hughes' big day brings an unexpected arrival.

Romance blossoms but a betrayal of trust threatens the chance of lasting happiness. Thomas considers desperate measures.

Mary endeavors to build bridges with her sister while Edith's secret continues to pose a threat. As Henry settles into the role of husband and stepfather, finding his place at Downton proves more difficult.