An extravagant house party at the Abbey gives the Crawleys a chance to reconnect with old friends, but some guests prove more welcome than others, and along with the celebrations come deceit and heartache.

Anthony Gillingham returns to Downton to propose to Mary, Edna tries to coerce Tom into marrying her, and Anna will not tell Bates what's bothering her.

Cora's playboy brother, Harold, is in trouble, so Robert must travel to America to help bail him out; a new farming venture at Downton sees Blake and Mary putting aside their differences.

Rosamund comes up with a plan to help Edith, but Violet sees right through it; Isobel is the object of someone's attention, while Mary asks Gillingham for a very big favor.

Summer of 1923. It's summer and as part of Rose's 'coming out' she is to be presented at Buckingham Palace. The Crawley family go to London to prepare Grantham House for this busy social program.

Tim Drewe guesses the parentage of Marigold, Mary receives another proposal from Tony, Violet plays matchmaker, Thomas continues to press Baxter for information with blackmail, and a flash fire threatens the mansion.

James leaves Downton, Mary has a weekend rendezvous with Gillingham, Carson and the Earl disagree for the site of the War memorial, a wireless becomes part of Downton life, and Edith becomes Marigold's godmother.

Mary wakes up in bed with Tony Gillingham after their tryst and returns to Downton, where she learns that a building firm want to buy land for a housing estate, to which Robert objects. Violet discovers from her butler Spratt about Mary and Tony but covers for her before summoning her for an explanation. Mary explains to her that she plans to marry Tony but later tells Tom the truth, that she has no intention. In return he confesses that he is considering a move to America. Following the policeman's visit, Anna is perturbed that her ordeal will be revealed, particularly as the animosity between John and Green comes to light. John gives his alibi, that he spent the day in question in York, and Mrs Hughes and Mary agree to collude with this. As Barrow goes to visit his dying father, Baxter tells Cora that she was beguiled into the jewel theft by a cruel footman called Peter Coyle, whom she loved but who fled leaving her to take the blame. Tim's wife wants to bar Edith from seeing Marigold, whilst Rose hosts a benefit for a group of Russian political exiles and Violet is taken aback by one of their number.

The Countess confesses her past with Prince Kuragin, Lord Merton proposes to Isobel, Rose's parents are divorcing, Lord Grantham becomes jealous of Simon, and Lady Mary tells Gillingham she won't marry him.

Violet's new maid doesn't seem to be working out, Kuragin reveals he still loves Violet, Anna realizes Bates' innocence, and Thomas reveals a secret to Baxter.

Atticus brings his parents, Edith and her daughter are brought home by Cora, Tom decides to take his daughter to America, Baxter volunteers to testify for Bates, and Isis is dying.

Kuragin tells Violet he wants to live with her, someone tries to break up the Atticus Rose wedding, Daisy wants to live in London, and Anna is accused of Green's murder.