In a world of "supernaturals," children of a supernatural and a human are called "probes." Probes are immortal but require skin-to-skin contact with an "echo" (the child of a "good" supernatural) to survive. Fearing probes, good supernaturals cursed them to have no gender or legacy, ensuring their line would die out. Now, human hunters track them down to finish the job. Amidst the hunt, a young echo named Sino has just become the target of a dying probe.