At the end of volume 1, the hordes of dead fish marching onto land with their tiny mechanical legs had spread from Okinawa to the coast of Japan. Tadashi's girlfriend Kaori had become infected by the gaseous excretions from the mobile dead fish, his uncle Koyanagi was doing what he could to help, and Tadishi himself passed out in a vat of what appeared to be minnows mixed with the occasional squid. Just the typical goings on for an Ito manga.
Tadashi comes to in a hospital and discovers that Japan along with the rest of the world is now overrun by the fish. A nurse tells him most people are becoming accustomed to the smell, but I find that hard to believe. Tadashi makes it across a Tokyo now under marshall law to his uncle's house. Uncle Kovanagi turns out not be so trustworthy after all, but what do you expect from a character drawn to look like Charles Manson. For the next hundred pages, Ito keeps upping the ante on weirdness and repulsiveness, OK, maybe none of it makes much sense, and scenes become somewhat repetitive and arbitrary, but Ito paints a convincingly nightmarish picture of what happens when fish with mechanical legs take over the land. He even creates a circus where infected humans are forced to perform for an audience of one.
Gyo has been made into an animated film. I missed the screening when it played a festival here in Dallas. Reports from those who both stayed up past midnight to watch it and made it all the way through were most enthusiastic.