A baby discovered in the Indian hinterland is cared for by a Japanese man. He is named Ricky and raised in Tokyo. However, Ricky was born with a large tail: surprisingly, he belongs to a different human race, the "Zero-Men," who are biologically close to squirrels but possess far greater power and intelligence than humans. Ricky meets his real parents, and they tell him that the Zero-Men live in an underground country in the Himalayas. They also reveal a dreadful plan: the Zero-Men will kill all humans in 20,000 hours so that they can take over the Earth. Ricky visits the country of the Zero-Men to meet the Grand Priest, who is their dictator and has an extreme phobia of humans. Tokyo has become their central base, where the Grand Priest's henchman, Enma Daio, is steadily carrying out the plan. Although Ricky is a Zero-Man, he has been raised in human society and decides to do everything he can to help the humans.