My biggest complaint with season 2 of OL is that the production staff seem to have no clue how to do plausible science fiction. They impose conditions and motivations into a script that ultimately prove to make the discerning viewer feel cheated out of a good story. After the excellent 'The Inheritors I and II', 'Keeper Of The Purple Twilight' avalanches right back into the banality of the rest of season 2.
It is a great title though, isn't it. It is never satisfactorily explained however. It really doesn't seem to mean anything perhaps.
So, the basic story is a dedicated scientist is seeking certain equations to help him complete his death ray and he is having no luck on his own. He is frustrated and emotionally sick over the stress of the situation. Along comes an alien to help but his motivations are bizarre. Basically he is supposed to provide the equations so that the death ray is produced on Earth and this would open the door for the rest of his species to come in and take over the world. It's sort of a warped hostile version of 'The Day The Earth Stood Still'. On top of this, the 'logical' alien advises the scientist he will provide the equations if he can have the scientists emotions. Well, I didn't see that coming - and, quite frankly, seems completely insipid. As you can surmise, the emotions don't sit well with the alien - blah blah blah.
I found myself wanting desperately for this episode to foray into something original or simply end. It finally ended. The only thing I managed to take away from this episode was the impressive alien configurations. These were worthy and could hold up to anything from the first season although that 'crushed velvet(?)' uniform was a bit laughable. And did I detect crotch zippers on the pants of the soldier aliens?? Did you see the claws on those guys? Logistically, urinating could be an adventure I would bet.
Overall: A dreadful episode slightly saved by the eerie alien costumes