At the beginning of season one, we find out about the death of two of Robert Crawley’s relatives on the Titanic. One of these relatives was also Downton’s heir, who was set to marry Robert’s oldest daughter Mary. Even though Downton’s money comes from Robert’s American wife, Cora, Robert and Cora can only leave their fortune to a male heir – and they have three daughters.
To ensure that their daughters will remain financially secure, a distant male relative, Matthew Crawley, is brought in as a promising match for Mary. Matthew is joined by his widowed mother, Isobel. However, Mary and Matthew are far from an ideal pair. Matthew dislikes the class distinction between the aristocracy and the working class that is so apparent at Downton Abbey, while Mary refuses to be forced into a marriage out of obligation.
Downstairs, Mr. Bates, who served with Lord Grantham in the military, is hired to serve as Robert’s new valet. This angers one of the footmans, Thomas, who had his eyes on the valet position. Thomas attempts to get rid of Mr. Bates after he learns that Bates was previously arrested for theft. When it is revealed that Bates went to prison to protect his ex-wife not only does he keep his job, but Mary’s maid, Anna, starts to fall in love with him.
Throughout the season, the butler, Mr. Carson, is blackmailed by his former music-hall performer partner; contemporaries Isobel and Violet Crawley clash over the local hospital; Mary must hide the fact that a suitor has died in her bed; and William the footman tries to woo Daisy, the assistant cook. Other storylines include Sybil convincing the chauffeur, Branson, into taking her to a political event that ends in violence, Mary derailing her sister Edith’s courtship, and Cora’s maid causing an accident that results in her having a miscarriage.
Thomas ends up joining the military after his failed attempt to oust Bates, and Matthew finally realises he has fallen in love with Mary, despite their bickering. Unfortunately, Mary does not share his feelings and rejects his marriage proposal. As the season ends, news of an impending war is spreading, after the assassination of Archduke Ferdinand.
As season 2 begins, William, Thomas, and Matthew Crawley are all fighting in WWI. Matthew and Mary continue to be friends, but Matthew is now engaged to another woman, Lavina. Mary admits to herself that she does in fact have feelings for Matthew, but it now appears to be too late. Mr. Bates’ ex-wife causes trouble for Bates and Anna’s budding romance, and Bates temporarily leaves his position at Downton. Sybil starts nursing training and Branson reveals that he has fallen in love with her.
William asks Daisy to marry him after learning he will die from a war injury. She obliges, even though she does not share his feelings. Matthew is also injured in the war and is told he will never walk again. However, after being cared for by Mary at Downton, he makes a slow recovery.
Mary is forced to accept an engagement to keep news of her dead suitor secret and Bates’ ex-wife insists she will not stop until she has ruined his life. Shortly after this threat though, she is found dead. Anna and Bates secretly marry, but Bates is arrested and convicted for his ex-wife’s murder.
During season 2 the war ends and Sybil and Branson announce their plans to marry, which shock Sybil’s parents. Lavina tells Matthew that they should cancel their engagement plans after she sees that Matthew and Mary have fallen in love. Shortly after this announcement, Lavina dies of the Spanish flu.
As the season ends, Mary escapes her engagement with the man blackmailing her, Sybil and Branson wed, and Mary and Matthew finally agree to marry each other.
As season 3 begins, Downton Abbey is in a financial crisis, as Robert’s investments have failed. While Matthew has enough money to save Downton, he is initially hesitant to use it; the money came from Lavinia’s father, who Matthew feels he deceived.
Sybil and Branson are now married and expecting a child, but Branson’s working class background and his strong political beliefs about his home country Ireland complicate his new role in the aristocracy. When Branson runs into trouble in Ireland, he is saved by his father-in-law, Robert. However, Branson learns he will face charges if he ever returns to Ireland.
Edith’s wedding day becomes an embarrassment as she is left jilted at the altar. She then finds sanctuary writing for a London newspaper. Anna continues to visit Bates in prison. He is eventually proven to be innocent and is released. Mrs. Hughes has a cancer scare and the hiring of the attractive young footman Jimmy causes a stir in the kitchen.
Tragically, Sybil dies during labor after an argument between two doctors concerning the proper treatment of her condition. Sybil and Branson’s daughter survives and Branson names her Sybil. Lady Rose MacClare, a young and somewhat wild Crawley relative, comes to Downtown to escape her warring parents, while Edith’s married newspaper editor declares his love to her.
As season 3 ends, Mary gives birth. Unfortunately, shortly after meeting his son, Matthew is tragically killed in a car accident.
Season 4 of Downton Abbey picks up six months after Matthew’s shocking death. Mary is still in mourning and struggling to show affection to her young son. Cora gets a new ladies’ maid, Edna, who takes an inappropriate liking to Tom, still grieving over Sybil’s passing. Edna leaves Downton after unsuccessfully trying to force Tom to marry her.
Lord Grantham grudgingly lets Tom and Mary work with him on much needed estate upgrades. Mary starts a flirtation with Lord Gillingham, but refuses his marriage proposal. When Gillingham’s valet, Mr. Green, brutally rapes Anna at Downton, she pleads with Mrs. Hughes to keep it a secret from her husband who she fears will seek revenge. Mr. Green later dies in a freak accident in London.
Edith spends the night with her married suitor, Michael Gregson, right before he departs to Germany to gain a divorce from his mentally incapacitated wife. Edith discovers that she is pregnant. With no word from Gregson after his departure, she decides to have her baby abroad with the help of the Dowager Countess. Rose has a brief romance with a black nightclub singer; Mrs. Hughes and Anna fear Bates may have had a hand in Mr. Green’s death; and Tom struggles to find his place at Downton and befriends a local school teacher.
As season 4 ends, Edith decides to give her child to a local tenant farmer to raise her so she can see her grow up. Also, it appears as if Gregson has been killed by violent men in “brown shirts” while in Germany.
As season 5 begins, Edith struggles to maintain a relationship with her secret daughter who is living with a local farmer. Robert and Carson are tasked with building a war memorial in the village and Tom’s new friend Miss Bunting causes quite a stir at Downton with her blunt political opinions.
Isobel receives an unexpected marriage proposal and Violet is reunited with a displaced Russian aristocrat from her past. Both women reluctantly turn down proposals of marriage and companionship.
Later in the season, Mary asks her maid Anna to procure her a birth control device so she can spend a weekend with Lord Gillingham. Soon after their weekend together though, Mary rejects Gillingham’s proposal and steers him to another woman.
Throughout the season, the police make several trips to Downtown investigating Mr. Green’s death and Anna is eventually arrested for murder. Thomas becomes ill and starts a treatment meant to cure his homosexuality. Cora is romantically pursued by a fellow art lover, which leads to a temporary strain in her marriage to Robert.
After Edith learns that Gregson has in fact been killed in Germany by violent men in “brown shirts,” she takes her daughter from the local farmer to raise her on her own. While Edith initially tells everyone that she had taken the child in as a ward out of charity, most of her family soon guesses the truth.
Tom decides to move to America and Rose marries a Jewish man, Atticus, whose father is at first very disapproving of Rose. Rose’s parents announce that they are going to divorce and Rose’s mother unsuccessfully tries to derail her daughter’s wedding.
As season 5, ends Mrs. Hughes accepts a marriage proposal from Mr. Carson and Anna is released from prison after a witness changes their story.