Hailing from the outer regions of space, from the Dark Gas Nebula in the Orion Constellation, a microscopic alien life form attached itself to the passing Ikeya Comet. This hunk of rock would eventually bypass Earth, and bring to the planet a new monster. However, the creature's arrival did not go unnoticed. While on a routine catch for shrimp, a fisherman, by the name of Gohei, discovers what appears to be a new breed of tadpole. Gohei immediately takes his finding to Professor Yano for study. However, before Yano can thoroughly study the specimen, a beast, resembling a giant version of the mysterious tadpole, engages two ships out at sea, sinking both in mere minutes. Fearful that their might be a connection between the sample and this new menace, Yano decides to investigate the common source of the occurrences: the nearby bay. The Professor dons his scuba gear, and submerges himself in the bay in search of clues, while his son, Ken Yano, awaits his return. However, while waiting for his father on shore, Ken is attacked by a bus sized tadpole. Ken fights back with his knife, tearing through the monster's stomach with no effect. Ken is left dumbfounded, though, as the mammoth tadpole spares him and returns out to sea. Once back underwater, the monster happens upon Professor Yano scuba diving, and is far less merciful. The Professor is brutally attacked by the monster tadpole, left with the right side of his face horribly deformed.
After Yano is brought back to his house, so that he can nurse his injury, the Professor immediately continues his studies on the new organism. Through his observations, Yano makes a startling discovery. As long as the animal is moist, and has an ample supply of pollution to feed on, it could grow indefinitely. However, Yano also discovers another horrifying fact. When the creature was cut in half, both halves where not only able to survive, but regroup and form a new larger body. It's this reason he speculated to his son, whom now dubbed the mysterious monster Hedorah after the sludge it lives in, why the beast was able to grow so fast.