Long ago, a civilization flourished on Mana, an energy supplied by giants known as the Luna, until a massive tsunami swept across the world and wiped it out. Ten thousand years later, the story unfolds underground. Mokugyo, a fish with plants growing from his back, and Toriiruka, a chatty creature, are two odd animals who cannot understand how they ended up there. Along the way they are joined by Harry, a lonely animal covered in needles, and Yona, a descendant of the ancients who has been charged with bringing civilization back to life. As the group wonders what really happened ten thousand years ago, the question remains whether Mokugyo and Toriiruka can make it back to the surface safely.