From the moment the Antares hands young Charles Evans over to the Enterprise crew -- nervously and in a hurry -- we know something isn't right with the boy. As the episode develops, we learn just how wrong he really is. He has been given a suite of psionic powers by the enigmatic Thasians, and he proceeds to misuse these abilities in a variety of ways.
The episode's strength is that it conveys the sense that literally anything can happen in space, and that a lot of it is likely to be beyond man's capabilities. But that's also, to a degree, a weakness. In the end, the fact that the crew had to be "bailed out" of their predicament, and the final fate of Charles Evans, detracted from an otherwise tense episode. As well, Kirk's last ditch effort to keep the boy with his own kind rang false, considering the desperation of his attempts to overwhelm and incapacite the boy immediately preceeding this. Surely, by this point, Kirk understood that Evans' lack of self discipline would have made coexistence with humans impossible.
The episode is further weakened by the fact that it is quite similar to "Where No Man Has Gone Before". In both cases, the central theme is Lord Acton's old maxim: power corrupts, and absolute power corrupts, absolutely. The subtle difference is that "Where..." clings to this belief, whereas this episode suggests it doesn't have to be that way; the Thasians have power superior to what Evans has been granted, and they do not (as far as we see) misuse it. In a sense, this shows as that social evolution is as important as cultural and technological evolution, a theme that appears from time to time in Trek. The human, Evans, despite the utopian Trek universe, misuses his power. But maybe, at some point, men will develop this power (as the Thasians did) and learn not to misuse it.
A rough and somewhat derivative plot, but with a strong central theme, good characterization and decent acting.