Erica, originally named Earwig by her mother, enters the story as an infant abandoned at St. Morwald's Home for Children. Her mother, a witch fleeing pursuit by twelve others, leaves her with a note promising eventual return. The orphanage matron renames her Erica Wigg, deeming "Earwig" unsuitable.

Growing up at St. Morwald's, Erica demonstrates exceptional cleverness and manipulative skills. She openly admits enjoying her ability to control others, stating she has "a whole army of minions at her disposal" among the children and staff. Erica expresses contentment with orphanage life, specifically praising the shepherd's pie and large sunlit windows. When her friend Custard questions her lack of interest in adoption, Erica dismisses the idea, believing only "weird" people would choose her and finding normal families too limiting for her ambitions.

Her circumstances change when she is adopted against her will by the eccentric couple Bella Yaga and Mandrake. Erica resists, declaring her wish to stay with Custard, but is compelled to leave. Upon arrival at their home, Bella Yaga reveals herself as a witch and states she adopted Erica solely for labor. Erica negotiates assistance in exchange for magic lessons, though Bella Yaga later reneges, assigning only menial tasks like grinding rat bones and gathering garden ingredients.

Erica discovers the house is magically sealed by Mandrake, preventing escape. Her exploration reveals hidden spaces, including a vast library and a garage containing a yellow car linked to her infancy. Inside the car, she finds a record album by the band "Earwig" and compares it to a cassette tape labeled "Earwig" from her orphanage belongings. She also learns Bella Yaga crafts spells for clients, ranging from sabotaging ballet careers to winning dog shows.

Frustrated by Bella Yaga's refusal to teach magic, Erica allies with Thomas, a talking black cat familiar. They secretly study Bella Yaga's spell book at night, crafting a resistance spell to nullify magical punishments and creating an "extra pair of hands" spell intended to free Erica from chores. The spell backfires when attached directly to Bella Yaga, escalating tensions.

Erica discovers connections between her past and her adoptive parents: Bella Yaga, Mandrake, and her mother were members of the rock band "Earwig." Her mother's departure from the band and subsequent witch pursuit form the unresolved backstory. The narrative concludes with Erica using her burgeoning magical influence to reshape household dynamics, inviting Custard for Christmas, and encountering her mother's unexpected return.