Former aquarium keeper Lin Yuan is electrocuted and reborn, turning into a koi fish about to be fed as live bait, plunging into a piranha tank infested with fierce fish. Facing the piranhas' encirclement, armed with human wisdom and knowledge of aquarium equipment, he unites cleaner shrimp, an old crab, and a host of fierce fish to break the law of the jungle in the fish tank, opposing aquarium owner Zhou Haisheng's illegal animal experiments. After the fish break through their cage and escape into the outer river, Lin Yuan refuses to be king, establishing a fish school alliance guarded by all races, using rules to guard their home, and pursuing true freedom in rivers without glass and iron nets.