A Lost Story/Script

I read, heard of a script that was being considered for the fourth season and was if anyone else had read or heard the same. I remember it from the 70's. The basic premise is that a landing party beams down to planet, Uhura is included, and they find a reverse pre civil war Southern US culture; where blacks are masters and whites are slaves. If anybody knows of this or has some idea (maybe a book) please let know. Thanks