The serious-minded, prim and prudish Charlotte Lovell shocks Delia Ralston with a revelation utterly beyond all imagination when the spinster tells her married cousin that a foundling at the orphanage where she works is actually her own child resulting from a secret love affair in the past. Because the man Charlotte is now about to marry has no idea the child is hers, she asks Delia to help rear the child so that she doesn’t lose contact with her daughter. The request is complicated by the fact that the father of the child was once loved by Delia herself who refused an offer of marriage on economic grounds. Nevertheless, her cousin agrees to help raise Charlotte’s daughter, Tina, but also assures that Charlotte is not able to follow through on her plans to marry.
Charlotte becomes more and more identified with the role of spinster aunt on her way to becoming the old maid of the title as Tina grows up and falls in love with Lanning Halsey. The relationship becomes unbearably similar to Charlotte’s relationship with Tina’s father in Delia’s mind so she takes steps to get move Tina into social circles. Delia moves to adopt Tina as a means gaining her entry into society so that she can marry rather get pregnant and become a disgrace like Charlotte.
The bulk of the rest of the story leads to its ambiguous ending in which Charlotte and Delia spar over who has earned the maternal right to tell Tina the facts of life on the eve of her wedding to Halsey.