Belinda opens with Belinda being criticized for her age and singleness. As her society dictates, she cannot remain an unmarried woman living alone, so she lives with her aunt. When her aunt can longer house her, the aging Belinda moves in with Lady Delacour, a charming older woman, and her husband. Belinda loves Lady Delacour, entranced by her grace and wittiness. They get along well, and the living arrangement works out beneficially for all involved for a while.
As time goes by, Belinda becomes suspicious of what Lady Delacour is not telling her. Seeing past the old woman's constant humor, Belinda figures out that Lady Delacour believes she is dying of breast cancer. Not only does she refuse to tell Belind but she's also hiding this suspicion from her family also. Confronting her benefactor, Belinda is accused of scheming to marry Lord Delacour upon the older lady's death. Lady Delacour becomes insensible, convinced that everything Belinda pretended to be was a lie.
No longer welcome with the Delacours, Belinda moves in with the nearby Percival family. She is treated well and has the opportunity to participate in a happy, healthily functioning family. Despite her improved circumstances, Belinda can't help but long to return to Lady Delacour's good graces. She admires the old woman and wishes to repair any damage in their relationship. Finally Lady Delacour sees a doctor and learns that her diagnosis is not terminal after all, though she is sick. She reconciles with her family, apologizing for not including them in her struggles before. And she invites Belinda to return. This time Belinda is able to help the family in profound ways, assisting the matron through her recovery process. By the end of the novel, the two unlikely women are solid friends and Lady Delacour has fully healed from her illness.