The family is at Mary and Matthew's wedding rehearsal where Lord Grantham expresses his approval that Sybil and Branson aren't coming. Isobel doesn't think there would be too much of a fuss, but Robert is convinced that Branson would be an object of interest to the residents of the village and he would prefer to allow more time for everyone, including the servants, to get used to the idea. Isobel presents her case to Countess Violet, who doesn't agree with her son, but does understand the importance of making Branson a non-issue. Isobel considers sending Sybil and her husband the money to make the trip herself.
Mrs. Hughes and Anna have returned from London where they were preparing the former Bates house to be rented out. Thomas is surprised the court allowed Anna to retain ownership of the house, despite her being married to a convicted murderer. Carson reminds him that Mr. Bates is still considered to have been wrongfully convicted.
O'Brien is keen to have her nephew hired as a footman now that Thomas is serving as Lord Grantham's valet and Carson will need the help. Carson would rather not have to train an inexperienced boy while preparing for the wedding. O'Brien goes behind Carson's back and makes the suggestion directly to Lady Cora who thinks it's a fine idea and presents it to her husband, who, after receiving an ominous phone call, distractedly tells her to do what she likes. He tells her he's going to London the next day. O'Brien's nephew, Alfred Nugent, is very tall and Carson isn't all that impressed with his work experience as a hotel waiter.
In prison, Anna visits her husband, giving him some papers she found that belonged to Vera. She wants him to identify any of the persons named so she can investigate whether or not Vera wrote or spoke to any of them about ending her life. On her next visit, he gives her his annotations, not convinced they will help, and encourages her to go with Mary on her month-long honeymoon to the south of France. She can bring back happy memories to share with him. His cellmate, a man named Craig, urges him to admit his guilt and give up on this scheme to prove his innocence. Bates warns him to keep his opinion to himself; Craig interprets that as a threat.
Matthew explains to Mary that he does understand Downton is her home, but he would like them to spend some time alone together in London so they can get used to each other without the whole family hovering around them. He'd also like to live more simply when they return. Mr. Molesley is disappointed when he realizes that he is not going to be serving as Matthew's valet at Downton, but relegated to being Mrs. Crawley's butler. Matthew impresses upon him how important he is to Isobel.
In London, Lord Grantham visits his financial firm where his lawyer, Mr. Murray, tells him that the money he'd invested in a Canadian railway, against their advice, has been lost. This includes the bulk of Cora's fortune. Without that money, running Downton is impossible. Since Robert's purpose in life is to maintain Downton as a major employer, he doesn't accept that he will have to sell off the property.
Edith accepts a ride from Sir Anthony Strallan, tells him her maternal grandmother is coming to the wedding, as are Sybil and Branson, though Lord Grantham doesn't yet know, and very much expresses her interest in resuming their relationship.
Upon his return, Robert is stunned to find out that a new footman has been hired, not remembering the conversation he'd had with his wife. Now facing financial ruin, he institutes a hiring freeze and starts fretting about the cost of his high-maintenance daughter's wedding.
O'Brien's opinion of Thomas grows even worse when he won't stop needling her nephew. Thomas moves on to Daisy who is dissatisfied that she was promised the position of Assistant Cook, but is still being used as the kitchen girl. Mrs. Patmore has told her about the hiring freeze, but Thomas still urges her to go on strike.
At dinner, Mary reveals that Sybil is coming to the wedding, while Alfred can't stop being a waiter and Carson wishes for a second footman. Matthew adds that he is hoping to live a more simple life after the wedding, but Countess Violet reminds him that they are responsible for providing gainful employment to the people in town.
Molesley frets about being relegated to Crawley House, especially when Thomas expresses his frustration at having to attend to Matthew, as well. With both Thomas and Carson flatly refusing to dress Branson, Alfred is recruited into that role, as well.
Matthew has heard from Mr. Swire's lawyer and believes that he may have inherited something from his late fiancee's father. It turns out that Swire rewrote his will on the death of his daughter to name three heirs, of whom Matthew is the last in line. One of the heirs has already died. The second left for India some time ago and hasn't been heard from since. Matthew's lawyer is making inquiries to try to find the man. If he's died, Matthew could inherit a ble fortune.
Sybil and Branson arrive. Lord Grantham denies having been the one who sent them the money and his relationship with his son-in-law has barely warmed. It doesn't get any warmer at dinner when Branson hasn't changed his clothing and gives his undiplomatic opinon about the Home Rule Act that divided Northern and Southern Ireland. His reception down in the servants' dining room is just as chilly, though he gets kind words from Mrs. Hughes and Anna.
Sybil explains to Mary how, in Dublin, there isn't the issue of the class distinction between the two of them, but, at Downton, Tom feels patronized. Mary assures her that time will improve everyone's treatment of him. She also warns Sybil that the Grey family has been invited for dinner the next night. Sybil is forced to tell her husband because Larry Grey, the son of Lord Merton, Mary's godfather, was once enamored of Sybil. She would also like Tom to hold his tongue about Ireland. Feeling judged by the family, Tom prepares to move into a room above the pub when Matthew finds him. He reminds Tom that he doesn't make it easy for the Granthams to accept him when he insists upon politicizing every meal. Since they are going to be brothers-in-law, married to stubborn Crawley women, he suggests they stick together.
Matthew tells Mary that his inheriting the Swire fortune is a longshot and he couldn't keep the money anyway even if he does get it. Lord Grantham finally confesses his business losses to Cora who sympathetically encourages him to find joy in their daughter's wedding, even if they'll have no more happy days in their home.
Alfred admits he'd wished he'd gone into cooking as it's an interest and talent of his, but O'Brien confirms that a male cook is hard for some to accept. Daisy would gladly accept him as a kitchen assistant. O'Brien has other ideas. Since Alfred will be assisting Branson, she'd like Thomas to give him tips on being a valet. Thomas reminds her how long and hard he worked to get the valet job here and refuses to help a newly-hired, inexperienced footman move up the ladder any faster.
After the Greys arrive, there is a pre-dinner reception during which Larry Grey makes several cutting remarks to Branson who storms off. While Matthew frets over replacing his ill best man, Sir Anthony, with Edith at his side, is speaking with Isobel about him coming to the wedding. He notices Grey with Branson's drink before he's distracted by Carson's announcing dinner. During the meal, Branson is at his most obnoxious, clearly inebriated, annoying everyone at the table except Larry whom Mary notices seems amused. That gets the attention of Sir Anthony who remembers Larry handling Branson's drink and accuses him of tampering with it.
Grey passes it off as a joke, but no one is amused, least of all his father who apologizes to everyone, including Branson, who is shuffled off to recover. Matthew decides to ask Branson to fill in as his best man. Seeing Sir Anthony off, Edith thanks him for saving the evening and kisses him on the cheek.
After the dinner guests leave, Cora is glad for the spirit of unity that Matthew showed that night. Robert admits they will all have to work together from now on. She encourages him to tell Mary, at least, about his money problems as she and Matthew will have to decide where to live after the wedding. She does, however, tell him it's best to leave her mother in the dark. Lord Grantham arrives in Mary's room, notes her expensive traveling clothes and asks the other ladies to leave. When alone, he tells her about his bad investments.
The next morning, Branson arrives at Crawley House where he apologizes to Countess Violet for his behavior the night before. Violet accepts with grace, then forces Branson to be fitted by Molesley for an alteration of one of Matthew's old morning coats.
Martha Levinson, Cora's mother, finally arrives at Downton, quick to comment on the rigid protocol, disparaging the overall resistance the English have to change and manages to both lovingly greet her granddaughters and rebuke them at the same time. At tea, she is eager to learn how the very distantly-related Matthew has the right to inherit her late husband's money. She's more impressed with Branson, who is bent on earning his own keep, before Mary realizes that it's the day before the wedding and her intended needs to take off.
During a brief moment alone, Mary tells Matthew about her father's uncertain future and asks about the Swire fortune, which could save them all. Matthew tells her that there is some evidence that the second heir has died, but the date of his death is important. If he died before Swire, Matthew inherits. If not, the man's heirs get the fortune. It doesn't matter to him either way as he will not keep the money, even if it means the Granthams are sent to the poorhouse. He refuses to profit off of a man whose daughter willed herself to death over him.
Mary accuses him of not really being one of them after all and storms off just as Edith appears in the hall.
Countess Violet arrives for dinner, trading barbs with Martha from the moment they meet. Over the meal, both Grantham and Cora deny being the ones to have sent the Bransons money before Violet admits she did because she felt Sybil and her husband should be there for the wedding. In any case, Tom is a member of the family now and Crawleys stick together. Mary admits that's not always true and leaves the table in tears.
Edith tells everyone she overheard part of an argument between Mary and Matthew, but isn't clear what it was about. Lord Grantham decides to go see Matthew, but Branson, being best man, argues it's his place to do it.
Mrs. Patmore allows Alfred to help her while shaming Daisy, who is standing there doing nothing, on Thomas' advice. Mrs. Patmore is aware of this and Daisy's strike doesn't last long.
Mary tells Anna what happened and Anna reminds her that Matthew has to be true to himself. Mary doesn't think that should mean putting himself above the family. Anna points out that men aren't like buses and she's not likely to be able to pick up another one in a few minutes.
Branson gives Matthew similar advice. He knew that Mary and Matthew were meant for each other even when he worked as a chauffeur. Neither of them will be happy with anyone else so long as the other lives. Matthew knows this is true and goes to Downton, knocks on Mary door and admits the same. Mary fears that this argument is indicative of many more if they cannot agree on this very significant issue. He doesn't want them to fight about something that hasn't happened yet and may never happen. He is still willing to marry her. But, so as not to break their luck, he keeps his eyes closed as he enters the room and kisses her.
The next morning, Lord Grantham still has trouble recognizing that Branson is no longer a servant, but thankful that he was instrumental in bringing the couple back together. Cora, Edith, Sybil and Anna help prepare Mary upstairs. Mrs. Hughes and O'Brien put the flower girls in the car before Mrs. Hughes checks with Mrs. Patmore. Mary appears at the staircase to the approval of her father and Carson. For the time being, Lord Grantham has forgotten his woes.
At church, Edith sits Sir Anthony with family as Matthew and Branson arrive. The grandmothers sit together. Mary is escorted down the aisle by her father and Matthew is relieved that she showed up.