Old Mortality is the story of a girl's search for meaning in her life as she grows up and learns about the life of her late aunt, who died young with a grand reputation.

Miranda first begins to learn about the life of her Aunt Amy when Miranda is eight years old. She sees a photograph of the beautiful woman, and she is filled with awe by her and hopes that she can be so beautiful one day. Miranda is told that Amy was a fine dancer, an excellent equestrian, and a wild, free-spirited person. Miranda's interest in Amy speaks to the romanticization of the past and what can happen to the image of a person who is only known by stories and visions.

Having died young, Aunt Amy is imbued with the characteristic of being forever dreamy and youthful, and she becomes an inspiration to the young and impressionable Miranda. The stories of her keep Miranda wishing that her life could be as grand as Amy's. Miranda's father also appears to be stuck in a bygone era—he constantly compares his two daughters to Amy.

Miranda both admires the stories of her aunt and ultimately realizes that she will never live up to these romanticized images of the past. This becomes particularly clear when Miranda is sixteen years old and runs into her cousin Eva. Eva, a devout feminist whose life has never been glamorized like the life of her dead sister, reveals to Miranda the real history of their family. She explains how the family has always been money- and fame-hungry and that their lives have been lived shallowly: chasing power and greed.

Miranda learns that her family is living in the past because they are obsessed with images of shallow beauty, money, and esteem. Miranda realizes that she wants to live a life in the present that has meaning for her that she can create for herself, as Eva has done with her own life.

Central to this story is the theme of encouraging young women to create their own meaning and expectations away from the moralities and perceptions of men. Miranda's father is always comparing his daughters to Amy, who is now nothing more than the representation of an era that is dead and gone. The character of Cousin Eva challenges this obsession with the past and with fanciful recollections.