The protagonist is the novel's first-person narrator, Stephen Stratton. The Passionate Friends is written as if addressed to Stephen's eldest son, who is on the verge of adolescence. Stephen is the only child of a rector who loses his faith due to Darwinism.
The most important relationship of Stephen's life is with the Lady Mary Christian (later Lady Mary Justin), a beautiful blue-eyed contemporary who has been his childhood "playmate" and with whom he falls deeply in love at the age of nineteen during the summer before he begins his studies at Oxford. Mary returns his love, but will not promise to marry Stephen. Having resolved to "belong to myself," Mary weds Justin, a wealthy financier, but intends also to remain Stephen's intimate friend.