Are you refering to the original 60's series or the make over version from the 90's (or both)? either way, their good.
Kind of reminds me a little of the Twilight Zone.
I find a good sci-fi story frequently has a mix of the following:
1) A philosophical issue to explore with a what if scenario.
2) A moral lesson that is sometimes connected to the philosophical issue.
3) A parallel reality that mirrors current political climates so readers can relate to the...