This alternative universe adventure follows Mirror, Mirror (1967), broadcast two months earlier. The premise is practically identical.
The anti-matter John behaves similarly to the Canto spirit John of season one's "Follow the Leader".
Almost a decade later a similar premise was explored on Seed of Destruction (1976). A mirror image of Commander Koenig comes to Alpha, leaving the original trapped behind. As with the crew of the Jupiter, suspicions are aroused when the doppelganger acts uncharacteristically domineering and stresses the need for immediate action.