O’Brien, leaving only a note to explain she has gone to work for Susan Flintshire in India, has snuck away from Downton Abbey, leaving Cora betrayed and without a lady's maid. Mrs. Hughes and Anna step in until a replacement can be found, and Rose, feeling responsible, puts a card in the post-office window. Edna Braithwaite applies and is hired on the basis of her glowing reference from Mrs. Hughes, who is horrified. She cannot dissuade Cora from making the hire without giving Tom away, and breaks the news to Tom and Carson. They have no choice but to let Edna come back and make sure she behaves herself.
Six months since Matthew’s death, Mary is still too deeply in mourning to take an interest in life or motherhood. Tom, with first-hand understanding of her grief, urges her to get involved in the running of the estate, but Robert vehemently disagrees, thinking they must shelter Mary from the world. Because Matthew didn’t leave a will, all but a third of Matthew’s assets now belong to baby George, and Robert, with his majority share, is adamant that he should be left to manage the estate. With crippling death duties to pay, Robert considers abandoning Matthew’s plans for Downton’s future.
Downstairs, the staff celebrates Valentine's Day, and Daisy and Ivy receive cards from secret admirers. Fuelled by Jimmy’s mischievous flirting with Ivy, the girls speculate as to who sent valentines to whom; Daisy desperately hopes that hers was from Alfred, but Alfred has eyes only for Ivy. Unable to stomach the silliness anymore, Mrs. Patmore eventually reveals that she sent it. Daisy is touched by Mrs. Patmore's motherly care but crushed that Alfred sent his valentine to Ivy. She is somewhat consoled by the arrival of a new electric blender, which she easily masters, much to the chagrin of Mrs. Patmore, who sees the appliance as a sure sign of her pending obsolescence.
Meanwhile, Mrs. Hughes notices Carson's agitation upon reading a mysterious letter. She retrieves it from the trash and learns that his old performing partner Charlie Grigg is in the workhouse. Visiting him, she's distressed to find him in squalor and despair, disappointed not to see Carson, with whom wants to make amends. Mrs. Hughes turns to Isobel to help the desperate man. But Isobel, broken with grief over her only child's death, has lost her sense of purpose. Or has she? The persuasive Mrs. Hughes suggests that Isobel take Grigg in and set aside her grief for another's good. Isobel agrees and Grigg is installed in her home. But Carson is furious to learn that Mrs. Hughes has read his letter and visited Grigg -- he doesn’t want to be reminded of a painful period of his life.
Sybbie and baby George's nanny, Nanny West, incurs Thomas' wrath when she reprimands him and gives him orders. She has created a dangerous foe, and before long Thomas confides in Cora his fears that Nanny West has been neglecting the children.
With Matthew gone, Molesley struggles to find work. Isobel has no need for a butler and Carson has asked him to leave Downton. Violet hatches a plan to convince her visiting friend, Lady Shackleton, to hire him, but at the luncheon she has arranges to show Molesley off, Violet's peevish, threatened butler undermines and humiliates him.
After months apart, Gregson invites Edith to a party in London. He shares his plans: although legally unable to divorce his lunatic wife in England, he could obtain a divorce elsewhere, like Germany. He is prepared to move there and hopes she will join him.
Tom's appeal to Carson to try to coax Mary back to life backfires when she accuses the butler of overstepping his position. Hurt and conflicted, Carson nevertheless makes the point that she is allowing herself to be defeated. At dinner, plans for the upcoming tenants’ luncheon are discussed, and Violet suggests that Mary preside, given that as George’s mother she will have to run the estate if anything happens to Robert. Overwhelmed, Mary storms out. After dinner, Violet seeks Mary out to impart some grandmotherly advice: Mary must choose between life and death. She encourages her to choose life.
Gregson's plans to become a German citizen are moving forward and Edith, intoxicated by her exciting life with him in London, finds it harder to resist her desire. As they grow closer, Edith invites him to an upcoming house party at Downton. Robert has little more than disdain for Gregson. Meanwhile, Cora overhears Nanny West in the nursery being cruel to little Sybbie. Appalled, she dismisses Nanny West on the spot and explains to Robert that they owe a great debt to Thomas.
On the eve of the farmer’s luncheon, Robert tells Mary to leave the running of the estate to him. His efforts to shield her give Mary pause and she seeks out Carson to offer an apology, breaks down, and is comforted by him. Reassuring her that she is strong, he reminds her that she owes it to Matthew to fight for the changes he made.
Mary surprises everyone when she joins the farmer’s luncheon. Though Tom is delighted, Robert’s face tells a different story. But just as she is emerging from grief, a box arrives from Matthew's office. Among the contents is a letter to Mary declaring her his sole heir. Everyone is overjoyed by Matthew's loving letter but Robert, who questions its legal standing.
At Crawley House, with no news from Carson, Grigg despairs that Carson blames him for causing him great unhappiness, but insists it wasn’t his fault. Isobel finds Grigg a job in Belfast and urges Carson to resolve their quarrel before Grigg leaves but he resists. Later, he reveals to Mrs. Hughes that he'd been mistreated by a former love, Alice.
Downstairs, Jimmy asks permission to take Ivy to the theatre in York, in spite of his last foray with her, when he allowed her to get drunk. Alfred is distressed that Jimmy is leading her on.
Meanwhile, Anna encounters Molesley mending a village road, humiliated and in debt. When he proudly refuses her offer of money, she turns to Bates for help. Bates gets Molesley's signature, forges a debt note, and then presents him with the long "forgotten" note and money. Confused, but unable to deny his signature, Molesley accepts the money. But Anna has a problem on her hands: she had advised Edna, on her arrival, to keep a distance from Thomas. When Edna accidentally damages a garment of Cora’s, Thomas drops a hint that it was Anna’s sabotage. She is Thomas' new ally, and Anna their new target.
Robert announces his lawyer's finding that Matthew's letter is legally valid: Mary now owns half of Downton. Robert takes the news well, considering Mary's spirited revelation that she disagrees with his plan to sell property to pay the death duties.
Mrs. Hughes informs Carson that Grigg will be leaving for Belfast in the morning and encourages him to let an old wound heal. Carson, reflecting on the picture of his lost love, Alice, arrives at the station just in time and hears Grigg's confession that he and Alice didn’t stay together. She had loved Carson. The old friends shake hands as they say goodbye.