Once in a Blue Moon, intended for all ages, is the magically realistic story of the Rainwater Family and their island in the middle of Good Bear Lake below Shadow Mountain in Colorado. Gabriel Rainwater is the family patriarch and Chris, his thirty-year old grandson. The Rainwaters are members of a clan of superhumans known as Rangers, whose mission it is to escort human spirits into and out of this world, and Gabriel is their head man. With Gabriel aging and Chris his only living relative, Chris believes he must be ready to be the next Rainwater head ranger and feels he must marry Lulu Big Sky, the daughter of the director of the ranger board of directors, Bob Big Sky, to strengthen their secret society. The difficulty is, Chris and Lulu don't love each other.
The trouble starts when the day before the wedding Amanda James, the human girl Chris really does love, comes back to town. This, at a time when Chris is under fire from Jack Newday, a non-ranger rabble-rouser, who believes Chris to be Bigfoot, a misnomer that has followed rangers like Chris for over a thousand years. The problem is, Jack is right. Chris is Bigfoot.