Over a century ago a comet struck the Earth, causing the sea level to rise and engulf the continents one by one. What’s left of mankind now lives aboard ships that float the endless ocean, and beneath the surface, giant, bloodthirsty giant sea creatures lurk, waiting to devour the flesh of divers that breach the surface. Bota and Lita are two young siblings that live aboard a scavenger vessel with their father, who tells the pair about legendary harpoonists who protected humanity from the monsters. And when tragedy strikes, one such harpoonist, Kana, takes them under her wing – but nothing can prepare Bota and Lita for the horrors that are to come…