Of all the episodes of Star Trek: The Original Series [1], of course the only one to make it to the stage is “Spock's Brain [2]” (link via Mike Sterling [3]).
No, really [4]!
[1] http://en.wikipedia.org/wiki/Star_Trek:_The_Original_Series
[2] http://mikecarano.com/startrek.html
[3] http://www.progressiveruin.com/2007_10_21_archive.html#9103423440258822585