Celestis

Celestis, inhabited by conservative human settlers fleeing from a socially decayed Earth, is riven by racial strife. Those conquered alien inhabitants who can afford it are surgically and pharmacologically altered to appear more human. Simon, a human diplomat, is taken hostage by alien rebels and thrown into contact with Katharine, an altered alien, with whom he falls in love. But Katharine, also a hostage, is cut off from her medications and rapidly begins the transformation back into a creature of alien needs and desires.