It involves a future when humanity is divided into two subspecies, or larities: Gens, who look like ordinary humans and produce excess quantities of a life energy called selyn, and Simes, who have tentacled arms and must take selyn from Gens in order to survive.
Children might become either Sime or Gen at puberty, and free Gens kill their children in self-defense if they start to change—if the child becomes Sime, it might take selyn from parents or loved ones, killing them.
Sime and Gen territories declare an uneasy truce, and channels strive to assist both sides.
Personal Recognizance / The Story Untold contains the essay "Chronology of the Sime~Gen Universe.
"[6] There is also an extensive collection of fan-fiction available online, particularly through the official fan site at http://www.simegen.com