An ode to Black girlhood and adolescence as seen through KB's eyes, What the Fireflies Knew follows KB after her father dies of an overdose and the debts incurred from his addiction cause the loss of the family home in Detroit. Soon thereafter, KB and her teenage sister, Nia, are sent by their overwhelmed mother to live with their estranged grandfather in Lansing, Michigan. Over the course of a single sweltering summer, KB attempts to navigate a world that has turned upside down. Her father has been labeled a fiend. Her mother's smile no longer reaches her eyes. Her sister, once her best friend, now feels like a stranger. Her grandfather is grumpy and silent. The white kids who live across the street are friendly, but only sometimes. And they're all keeping secrets. As KB vacillates between resentment, abandonment, and loneliness, she is forced to carve out a different identity for herself and find her own voice.