The crew encounters Nomad, a computerized Earth probe that somehow merged with an alien probe and subsequently launched a mission to "sterilize" (read: destroy) anything that is "imperfect." Nomad has already killed millions. Fortunately, Nomad mistakes Kirk for its creator, a scientist who died hundreds of years ago. This gives Kirk just enough perceived authority over the machine to keep it from destroying the Enterprise and its crew.
Nomad and the mystery behind its existence is neat in story terms, and it being on the verge of destroying everything keeps us mindful of the danger. A scene where Spock mind melds with Nomad is interesting (even though I wondered how he could read the thoughts of a computer). But the episode suffers from a few too many unproductive gimmicks: Scotty dying and then undying; Uhura's mind being wiped of all information; and, of course, the cliche where Kirk Outsmarts the Computer™ yet again—although this time it seems a little more plausible than in previous episodes.
Unfortunately, the ridiculously implausible idea of Uhura's wiped mind being retrained with basic education (she is reading sentences on the level of "See the dog run" at one point in the episode) is more than just a little absurd. The fact that she's on the bridge the next week as if nothing happened is just plain silly.