Offering a view of English life from New Year's Eve 1899 through New Year's Day 1933, the film is presented from the point of view of well-to-do London residents Jane and Robert Marryot. Several historical events serve as background for the film, including the Second Boer War, the death of Queen Victoria, the sinking of the Titanic, and World War I.
Seeking background material for a mystery he is working on, novelist Charles Condomine (Harrison) invites eccentric medium Madame Arcati (Rutherford) to his home in Lympne, Kent, to conduct a séance. As Charles, his wife Ruth (Cummings), and their guests the Bradmans barely restrain themselves from laughing, Madame Arcati performs peculiar rituals and finally goes into a trance. Charles then hears the voice of his dead first wife, Elvira (Hammond). When he discovers that the others cannot hear her, he passes off his odd behaviour as a joke. When Arcati recovers, she is certain that something extraordinary has occurred, but everyone denies it.
While en route to Paris via train, commercial artist Gilda Farrell meets artist George Curtis and playwright Thomas Chambers, fellow Americans who share an apartment in the French capital. Gilda works for advertising executive Max Plunkett, who has had no success in his efforts to engage her in a romantic relationship. Tom and George each realizes the other is in love with Gilda, and although they agree to forget her, they cannot resist her when she comes to visit. Unable to choose between the two, she proposes she live with them as a friend, muse, and critic—with the understanding they will not have sex.
In the latter months of 1938, Laura Jesson (Celia Johnson), a respectable middle-class British woman in an affectionate but rather dull marriage, tells her story while sitting at home with her husband, imagining that she is confessing her affair to him.
Larita (Jessica Biel), a glamorous American widow and successful racecar driver, meets young John Whittaker (Ben Barnes) in Monaco. They marry, and he takes his bride home to meet his family at their dauntingly large rural mansion, where seven generations of Whittakers have been gentleman farmers. There she meets her icy cold mother-in-law, Veronica (Kristin Scott Thomas) and disheveled, sad-eyed father-in-law, Major Jim Whittaker (Colin Firth).
Opening in 1919, shortly after the end of the First World War, the film focuses on the middle class Gibbons family after they settle in a house in Clapham, South London. The household includes Frank, his wife Ethel, their three children – Reg, Vi and Queenie – his widowed sister Sylvia and Ethel's mother. Frank is delighted to discover that his next-door neighbour is Bob Mitchell, a friend from his days in the army.