Unter Wilden (1972)

A tribe of primitive "mudpeople" encounter a croquet ball, rolling through their forest. Following it, they find themselves on a vast, deserted Long Island estate. Entering, they begin to become civilized and assume the stereotypical roles and dress of people at a weekend party. There follows an allegory of upper-class behavior. At last, they begin to devolve toward their original status, and after a battle at croquet, they disappear into the woods.