On Whetsday — Mark Sumner

Generations ago, the cithians of planet Rask rescued Denny's ancestors from a disaster that rendered Earth uninhabitable. Since that time, the cithians have cared for their human guests, feeding them and housing them and expecting nothing in return except obedience to a few simple rules. The generosity of the cithians is all Denny has ever known. He never doubted their good intentions until the day he danced into a secret -- a secret that could mean death for Denny and his small adopted human family...and the end of humanity forever.