Investigating what is apparently a "rip in space," a landing party beams down to a planet to find Lazarus (Robert Brown), an unstable man involved in a mysteriously bizarre problem: He's in a battle with his counterpart self from a parallel universe.
Among a stretch of shows that exemplifies many of Trek's most visible qualities is the arrival of this episode, which unfortunately exemplifies science fiction excess. First of all is the ridiculously extreme notion that the meeting of the two Lazaruses (or is that Lazari?) would mean the destruction of the "entire universe." Such overlarge devices are rarely effective. Also, this episode seems to be in love with its own use of sci-fi buzzwords. In addition to matter and antimatter, we've got the concepts of a "parallel universe," a "rip in space," a "time ship," an "inter-universal gateway," etc. Little of this makes much sense, no matter how hard Shatner and Nimoy try in bouncing incredulous dialog off each other.
The episode becomes an untenable collection of disjointed story items with no overriding cohesion. (And, by the way, why would the Enterprise destroying one time ship cause the parallel universe time ship to be destroyed?) Saving some face is the somewhat interesting implication of Lazarus fighting his duplicate counterpart "for all eternity."