The original concept is the same as the manga, however, the story itself differs quite a bit. First off, the focus of this OVA is on the female protagonist, Kaori, rather than the male protagonist, Tadashi, as in the manga. The story follows Kaori's quest to reunite with Tadashi (who is pushed firmly into a supporting role) in the midst of a crisis. What exactly is this crisis, well... an invasion of walking fish. Yes, feel free to laugh now.
Despite the complete absurdity of the idea of a mass scale invasion of walking fish, Gyo manages to work decently with it, albeit the viewer is required to take it with a grain of salt. There actually is a real sense of danger, with people being devoured by swarms flesh eating fish or great white sharks. Scenes of mass panic juxtaposition with graphic gore and people freaking out, give the OVA a frantic and chaotic atmosphere that should keep you engaged. Or if that does not work, the sheer absurdity of it all will keep you equally as entertained.
Kaori's struggle to reunite with Tadashi also makes for a compelling watch. Her desire to reunite with Tadashi and her worry for him is easy to empathize with; after all, who wouldn't be concerned their loved-ones in the midst of mass crisis, even if it is something as ridiculous as an inland invasion of sea-life. The sub-plot involving Kaori's two friends provides a nasty little denouncement of friendship. From the first few seconds with them it is clear that their friendship is paper thin and held together by Kaori alone; so when she leaves to find Tadashi in the middle of the crisis... things get ugly. Finally, there is the role reversal from the manga between Kaori and Tadashi near the end, which might get a few snickers from those who read the manga, but more importantly adds gravity to the stories conclusion, albeit not gracefully.
This all is not to imply Gyo's story is a well oiled machine, far from it. The reason for this horrific anomaly is not particularly well developed, nor does it make complete sense. There are plenty of unexplained plot conveniences like Kaori's friends suddenly being in Tokyo. The story also goes out of its way to show you the most unpleasant of things in the most unpleasant ways. With a graphic sex scene and even more graphic mutations, people will be justified in saying this is too excessive. Some may even find it comedic in the extremes it goes to.
Although the grotesque and bizarre content of the manga comes off a bit awkward or overly disgusting (though it is just as disgusting in the manga) in the transition to animation, Gyo is still solid on the technical side. Character designs are cleaner, though somewhat less detailed than Ito's original designs. However, they don't differ too much and it is easy to see the resemblance to the original artwork. The invading swarms of fish are rendered in CG; while it certainly is not the best CG integration ever, it still looks plenty better than the attempts of many an anime. The grotesque mutation body horror and gore are definitely what get the most attention here, enough to possibly nauseate some viewers; this is of course the intent, for better or worse. The music is what you might expect in a horror, haunting and tension filled. It fittingly supports the grotesque abnormalities you see on the screen.
Ultimately, Gyo might be too gross and weird for a lot of viewers to enjoy. Those who find it repulsive certainly would not be unjustified in doing so. Still, it does exactly what a B-horror should do, make the audience reel back in shock and disgust... well, that or laugh at the absurdity of it.
