Paris, 2045. The city is controlled by Alma, an artificial intelligence, and divided into three zones, each inhabited by a separate class of population. When the creator of the artificial intelligence is murdered, the crime must be investigated by Salia, an investigator from the second zone, and Zemu, a police officer from the third zone.

An old fisherman brings Professor Yano a strange creature resembling a tadpole and casually complains that his catch has dropped sharply. Soon, the TV news shows something gigantic breaking a ship in half, and the professor's son Ken finds a resemblance to the fisherman's "tadpole." The professor and his son set out to explore the coast. They discover a giant "tadpole." Ken names the creature Hedora. Further study reveals that Hedora feeds on pollution from the bay, and small Hedora tadpoles can form into a giant Hedora, which can take any shape and even fly. Soon, the giant Hedora appears in Sagami Bay. The monster climbs onto the pipes of a local factory and, like a heavy smoker, inhales smoke from them. Godzilla challenges the new monster. People begin to die in the battle between the two titans. Who will stop the monster?