Imagine that there are parallel worlds. One world is the world that everybody knows. The other is a place of pure imagination-- populated by the myths, legends and creatures of lore that humans have thought about since the beginning of time. In the beginning, this parallel world is a dark and wild place and the creatures in it are gods and demons-- awesome and mighty. Imagine the consternation that mass media brings: Suddenly, the Devil is face-to-face with Mickey Mouse. When that happens, there is Hell to pay...
Out of Their Minds is a charming conceit, but truthfully not one of the best of the Simak collection. I have kind of the feeling that he had a terrific idea, but never worked it out very far past the point of being a terrific idea. The introduction is great, but the follow-through is a pale shadow of some of his greater novels like Way Station or Destiny Doll.
Simak fans should find this a nice digression (I love the moment with Snuffy Smith!) but people new to his work should begin elsewhere.