Nannion tells the story of a gigantic experiment in marine biology and evolution. After World War II, the inner part of a small island in the Greek waters was carved and filled with sea water to create the largest aquarium in the world, allowing the study of complex marine life like it had never been possible before. The project was eventually abandoned, but life continued to evolve, as it does in almost every ecosystem. Decades later, a small group of scientists visit to investigate evidence of large lifeforms. The island is also home to a small, hapless cat that was inadvertently stranded months before. The scientists discover bizarre lifeforms and ecosystems as they gradually realize there is danger on the island. Inter-species communication becomes key to survival. The ecosystem is changing and so are some denizens of the island. As they fight for survival, they also strive to define their own identity.