Synopsis:

A freak transporter action causes a juxtaposition between Kirk, Scotty, and Uhura and their counterparts in a parallel universe.   This Universe though is governed by terror and evil.

This episode demonstrates its impact by being used for future episodes in both TNG and DS9 plus being the basis for a couple of books.   The alternate universe is somewhat believable but there are problems with how one set of personnel follow it and others are not included at all.   Still the show makes for some very entertaining viewing.   This episode convinced me that Mr. Sulu could easily have missed his calling and possibly should have been a medieval inquisitor.