An example of a good idea gone bad, this candidate for "worst Star Trek episode ever" has some interesting ideas that are buried beneath layers of confusing monotony.
The basic premise, that a man is really two individuals switching between mirror universes is an interesting idea and only half a step away from second season's classic "Mirror Mirror" episode. In this case, however, the writers never develop much of a plot around it. (They tried to, with a relationship between Lazrus and Lieutenant Charlene Masters, the African American engineer, in an early draft. The same storyline, however, had already been put into motion for "Space Seed" by the time this one was being written, and the subplot was dropped).
To make matters worse, on the first day of the shoot, guest star John Drew Barrymore (cast as Lazarus) never showed up, forcing the cast to shoot without a guest star for a day before another actor, Robert Brown, was brought in, rushed through wardrobe and make-up, and then dragged onto set as well as out to the Vasquez Rocks, used again as an outdoor location.
The end result is an episode that consists of basically the same eight minutes over and over again: Lazarus, with poor clothes and inconsistent make-up, wonders around, goes mad, the universe begins winking out as two stuntmen fight, and then Kirk asks Spock what's going on, to which Spock has no answer. Throw in the first mention of dilithium crystals (replacing the "lithium crystals" mentioned in earlier episodes) and what appears to be George Jetson's car, and you get the worst episode of Star Trek's first season.
What's sad is that the plot itself, as well as the climax, have some thoughtful issues that could make for a solid foundation to a good science fiction tale; but the elements don't come together here the way the show originally hoped they would.
Remastered: Knowing there's no reason to go out of their way this one, CBS keeps it's basic here, replacing another appearance of old Big Red with an upgraded planet, and giving us some basic Enterprise beauty passes. Lazarus's eternal struggle with his enemy gets a slight upgrade as well.