Though her trip to Okinawa was supposed to be a lovely excursion to the seaside, college student Kaori Sawahara finds that it is anything but when a horrifying menace emerges from the watery depths. A strange mechanism has taken control of the sea creatures, granting them legs and sending them on land to attack anything in their way. Kaori and her friends—the lustful Erika and the reserved Aki—find themselves caught up in this onslaught, made worse by the creatures' rotting stench of death.

Surviving the first assault, Kaori tries to contact her fiancé Tadashi in Tokyo. However, the call is cut short in what sounds like another fish attack. Fearing for his safety, Kaori leaves for Tokyo. With Erika and Aki contending with the attacks in Okinawa, Kaori rushes to find Tadashi, unaware that the crisis has only just begun.

Gyo adapts Itou Junji's manga series of the same title. It follows the same story as the manga but switches the roles of Kaori and Tadashi while adding new characters and subplots.