Walthros is an alien planet where all life resembles Earth animals. Long ago, a legendary hero ended a world-spanning conflict by unleashing a power that destroyed most of the known world. Thousands of years later, the land has largely recovered — but old conflicts have been reborn, and war continues. Bob Surlaw, a Walthrosian Fish who makes a living exploring ancient ruins, leaves home one morning to investigate a nearby shrine that has been sealed for generations. Inside, he finds an angry spirit and a fragment of an ancient relic — the start of something far larger than an ordinary expedition.