April 1919: As preparations are underway for Matthew and Lavinia's wedding, Lady Sybil's expressed wish to marry Branson shocks and horrifies her parents. Lord Grantham is violently opposed to the idea but his threats of social disgrace and disinheritance do not dissuade Sybil. Lord Grantham then attempts to bribe Branson, but Branson's intentions are sincere so he refuses.
Cora, Carson, and Lavinia are taken ill by the Spanish flu. Thomas' black market venture was a spectacular failure, leaving him penniless and without a job but Carson's illness gives him a chance to become useful in the house.
While Lavinia is resting upstairs, Matthew finds himself alone with Mary and tells her what Violet said to him: that he should marry her. Both of them acknowledge he could not as it would be terribly cruel to Lavinia. However, they kiss just as Lavinia is coming down the stairs. When her illness delays the wedding, Lavinia tells Matthew that she heard and saw everything. She feels they should end their engagement but Matthew refuses. Fearing Lavinia’s illness may bring together Matthew and Mary, Richard Carlisle comes to Downton Abbey.
Daisy is distressed by William’s father’s request to visit him. Ethel is surprised to learn that Major Bryant’s parents want to see her but his father wants to take her child away permanently. Eventually, she decides to keep the child.
Lord Grantham finds himself unable to control his desire for Jane, who willingly responds. However, their night-time encounter is interrupted and remains unconsummated; and Jane decides to leave. With the possibility of more trouble regarding Vera's death, Anna insists that she and Bates should marry so that she may be able to support him through his difficulties as his wife. They marry in secret, with Lady Mary's permission and she arranges for them to spend their wedding night in a guest room.
Cora becomes seriously ill and Clarkson fears the worst. O’Brien tirelessly cares for her and tries to ask for her forgiveness. However it is Lavinia who succumbs to the flu and dies saying that it is best for Matthew. Wracked with guilt, Matthew tells Mary that any relationship between them is now impossible. Somewhat chastened by the recent events, Lord Grantham reluctantly gives his blessing to Sybil and Branson. The episode ends with Bates’ arrest for the murder of his late wife.