Stephen Gordon is the female protagonist of the book; given a boy's name at birth by parents who were expecting to have a boy, and christened their daughter with the name they had already chosen, Stephen is born in the late Victorian era in England, into an upper class family of means. She hates being a girl, and everything to do with being a girl. She has an androgynous physique even as a baby, She wants to be a boy.
When Stephen turns seven years old she starts to notice women, specifically the family's housemaid, Collins, on whom she develops a crush, her first one. Her misery is unconfined when she spots Collins and a footman sharing a kiss one day.
Stephen's father absolutely adores her, and wants to understand her. In order to do this he turns to books written by Karl Heinrich Ulrichs, the first writer to develop academic theories about homosexuality. By contrast, Stephen's mother, Lady Anna, feels that there is something wrong with her daughter and their relationship is very distant, by her mother's choosing. She also feels that in some way she has sullied the ancestral line.
When Stephen is eighteen years old, she becomes very close to a young man, a Canadian called Martin Hallam. She is very fond of Martin but the love she feels for him is strictly platonic, possibly even sisterly, but Martin has more sexual feelings for her, and when he voices them, Stephen is horrified. The friendship is ruined; more loss follows the following year Sir Phillip is killed in a freak accident when he is crushed to death under a falling tree. As he dies, he tries frantically to explain their daughter to his wife, wanting to let her know that Stephen is homosexual, but he does not manage to do so.
Now an adult, Stephen begins to dress in men's clothing, visiting a tailor for her bespoke clothing instead of the dressmaker that her mother recommends. She falls in love with an American woman named Angela Crosby. Angela is married and she and her husband are neighbors of Stephen. Angela is intrigued and amused by this relationship, thinking of it as more of her schoolgirl crush come to life, rather than as a relationship that has a future to it. A bored housewife, a lesbian affair adds a little interest and diversion to her day. Stephen soon learns that Angela is also having an affair with a man, which scares Angela as she is frightened that Stephen will say something to her husband. She preempts this possibility by showing her husband a love letter that Stephen has written to her, and her husband promptly sends it to Lady Angela. Lady Angela is angry and tells Stephen that she should never use the word love to describe the unnatural feelings she is experiencing, and that she should exert more control over her physical urges. Stephen tells her that she loves Angela in the same way that her mother loved her father. Their argument is heated and afterwards Stephen takes refuge in her beloved father's study.
She peruses his locked bookcase for the first time, finding a book by Krafft-Ebing, which is about homosexuality and sexual deviation. She recognizes herself in the book and realizes for the first time that she is a homosexual.
Stephen moves to London and writes her first novel which is very well received. Her sophomore novel does not do so well, and so at the urging of her friend Jonathan Brockett, who is a homosexual playwright, she moves to Paris to improve her writing and to broaden her horizons. She meets a circle of lesbian friends including salon hostess Valerie Seymour. World War One breaks out whilst she is in Paris; she joins the ambulance corps, and subsequently serves on the front line earning the Croix de Guerre, the French equivalent of the George Cross or the Purple Heart. She also falls in love during the war; the object of her affections is a fellow ambulance driver, Mary Llewellyn, who moves in with her after the end of the war. At first, things go well, but when Stephen returns to writing, Mary starts to feel lonely. Because of her orientation, she is largely shunned by society's elite and so throws herself into Parisienne nightlife where she feels far more accepted. Stephen is very conscious of her role in this change in Mary's life, and also feels that their relationship is making Mary bitter and cynical.
Martin Hallam has also moved to Paris; he and Stephen become friends again, and he also develops very strong feelings for Mary. Stephen has convinced herself that she cannot make Mary happy, at least, not entirely, and so pretends that she is having an affair with Valerie Seymour so that Mary will seek solace in Martin's arms.
