Mamoru (Tadanobu Asano) and feckless, unstable Yuji (Jô Odagiri) are best friends who work together in a Japanese towel factory. But they're separated when, after lending Yuji a poisonous jellyfish, Mamoru is jailed for killing their boss (Takashi Sasano). Suddenly at loose ends, Yuji begins working for Mamoru's father, Shin-ichiro (Tatsuya Fuji), in his salvage shop. Meanwhile, the jellyfish escapes and begins reproducing in massive numbers within the sewers of the city.