Confessions of an Ugly Stepsister tells the story of Iris, the plain younger daughter of Margarethe Fisher, as she takes care of her mentally challenged older sister Ruth and her beautiful stepsister Clara. Having fled from the Fens of Cambridgeshire, England to Haarlem, the Netherlands, upon her father's death, Iris is slightly at odds with the world and often contemplates the value of beauty and ugliness. While caring for her sisters and keeping the peace between Clara and Margarethe, Iris develops a painter's eye and spends time studying under a local painter known as The Master, and his apprentice, Caspar.
Margrethe makes Iris and Ruth go to the ball in the hopes of making the prince fall in love with Iris. Iris secretly helps Clara get to the ball and the prince immediately falls in love with her. While at the ball, Ruth does the unthinkable out of jealousy and love of Clara and Master, burning down the Master's magnum opus, a painting of Clara. That night, the fairy tale of Cinderella and her pumpkin carriage is spun, and the next morning her prince comes to collect her.
At the end of the tale, the characters' eventual fates are revealed: Iris marries Caspar and paints at his side, sometimes under his name; Caspar "dutifully" cares for Ruth; and Clara eventually dies in New Amsterdam from a complaint of the heart.