A baby found in the Indian hinterland is looked after by a Japanese man. He is named Ricky, and raised in Tokyo. But Ricky was born with a big tail: surprisingly, he belongs to another human race, the "Zero-Men," who are close to squirrels biologically but have far greater power and intelligence than humans. Ricky meets his real parents, and they tell him that the Zero-Men are living in an underground country in the Himalayas. And they disclose a dreadful plan: that 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 who has an extreme phobia to humans. Tokyo has become their central base, where the Grand Priest's henchman, Enma Daio, is steadily carrying out the plan. Though Ricky is a Zero-Man, he has been raised in human society, and decides to do everything he can to help the humans. (Source: Tezuka Osamu Official)