Based on the 1981 book of the same name by new age guru Ken Keyes, Jr., which in turn is based on the story of a 1952 experiment on the southern Japanese island of Kojima. Once one monkey has learned how to wash its food, it is able to teach others the same skill. When a certain number have learned (and Keyes suggests an arbitrary figure of 100), not only do the remaining monkeys suddenly appear to know how to do it, but so too does a completely different group of apes on a completely separate island. Keyes uses this apocryphal tale as a parable for the antinuclear movement, as "proof" that change would come if enough people joined forces. (Source: The Anime Encyclopedia)
