Imagine you are quietly minding your own business and living your slightly messy life when you receive a telephone call that turns everything you ever knew about your family upside down. The caller says your elderly great-aunt is being released from an asylum where she has spent most of her life, and you are the only relative who can take responsibility for her.

No one in your family had ever mentioned this person. In fact, your grandmother, her sister, has always insisted she was an only child.

This is what befalls 31-year-old Iris Lockhart of Edinburgh, Scotland, in “The Vanishing Act of Esme Lennox” by Maggie O’Farrell. The asylum where her great-aunt Esme has lived since she was 16 is being closed. Iris’s grandmother, Kitty, has Alzheimer’s disease, and gave the asylum staff Iris’ name in case of emergency.

Ms. O’Farrell does a masterful job of unwinding the story of how Esme got where she was and why she was essentially left there to rot. Although this is a work of fiction, the author was inspired by true cases of women committed on the word of a single doctor for behavior as innocuous as taking long walks, refusing to marry and dancing around in front of a mirror in their mother’s clothes.

The story flits back and forth between the present, Esme’s girlhood and the aftermath of her commitment. It can be a challenge to follow the thread, but once you get the hang of it, the rewards are huge.

Esme and Kitty were born and raised in India to a pair of stiff-necked colonials living in the last glow of the British Empire. Esme had always been a “wild child,” while Kitty, six years older, was obedient and compliant. When Esme was 4, a brother, Hugo, was born. Esme adored him, but while her parents and Kitty were away one weekend, typhoid swept through their home, killing Hugo and their Indian nanny. The servants abandoned the home in fear of the disease, leaving Esme alone with the dead.

That trauma would be enough to unhinge anyone, and it is only the first of a string of personal catastrophes for Esme. But is she really mad? Or just a bother to her family?

Iris’ own personal history provides a 21st-century echo to her aunt’s. The daughter of Kitty’s only child, Robert, who died young, she was raised by her mother, Sadie. When Iris was 5, Sadie fell in love with a man named George, who moved into their apartment with his son, Alex. Although Sadie and George broke up several years later, Iris and Alex have had an intense, sometimes sexual, relationship all their lives. Even though he is married and she is dallying with a married lover, it is obvious they are more interested in each other than anyone else.

The third major character in the book is Kitty, who recalls snatches of the story through the veil of her dementia. Ensconced in a pleasant nursing home, she tries to justify unforgiveable actions to the reader. The climactic scene in the book is a reunion of the two sisters.

The nature of madness, eccentricity, social norms and laws governing the mentally ill made for a lively discussion when “Esme” was the focus of the Remainders Book Club at Ooh La Lolly last week. Everyone had a slightly different take on the book, but we agreed it was one of the saddest stories we had ever read. Often when I finish a particularly complex book like this one, I will go back to the beginning and re-read it to pick up on things I missed. I started to do this with “Esme,” but had to stop. Even so, the story and its characters haunted me for days afterward.