In 2029, a series of events sent the country into civil war: the government fell, cities were destroyed, and millions of people perished. Those who survived went into hiding. This event is known as the Change, and everything that came after—the Beyond.

Sixty years later, Nessa Aisling wants three things: to be treated equally to the men of her division, to venture outside, and answers. She has lived her entire life in hiding, trapped within the walls of her division. On her 18th day of aging, Nessa finds limited freedom as Guardian but struggles to accept her place in the division. She pushes back against the Elder council's oppressive views, strict traditions, and refusal to intervene with those in the Beyond.

Raised not to ask questions, Nessa searches for answers in a country that no longer exists and begins to question if the true dangers are in the Beyond or within her division. In her desperate pursuit of purpose and truth, Nessa unknowingly sacrifices everything.