Set in Edwardian London, Night and Day contrasts the daily lives and romantic attachments of two acquaintances, Katharine Hilbery and Mary Datchet.

The novel has four major characters: Katharine Hilbery, Mary Datchet, Ralph Denham, and William Rodney.

Motifs throughout the book include the stars and sky, the River Thames, and walks.

After a time they end their engagement so that Rodney can explore a relationship with Katharine's cousin, Cassandra Otway.

Ralph Denham is a lawyer who occasionally writes articles for a journal edited by Trevor Hilbery, Katharine's father.

Unlike a few other characters in the novel, he has to work to make a living and take care of his family: his mother, a widow, and several siblings.

Mary Datchet, the daughter of a country vicar, works in the office of an organisation that campaigns for the enactment of women's suffrage.

He is Katharine's first romantic interest, but he is largely attracted by her grandfather's status as one of the greatest English poets.

The story, though it centres on Katharine, Ralph, Mary, and William, is dotted with minor characters who appear for the most part only at the various tea parties.