Art Wallace’s screenplay (the story is credited to Wallace and to Gene Roddenberry) has some playful moments and some high suspense. In fact it’s a sort of spy thriller.
It all starts when Gary Seven (Robert Lansing), in the process of being beamed by transporter beam over an unimaginable distance, gets caught in the Enterprise’s transporter beam. Gary Seven appears to be a perfectly ordinary twentieth century human, but if that’s what he is how could he have been transported across a distance of thousands of light years? He claims to be what he appears to be but explains that he’s been living for some time on a much more advanced planet, a planet the existence of which is totally unknown to the Federation. He also claims to be on a vital mission to Earth, The fate of civilisation might well hang in the balance.
His story, however outlandish, might be true. Or he might be some kind of alien in human form. Kirk has no way of knowing but he must decide whether he should be helping Gary Seven or stopping from doing whatever he plans to do.
There’s some nice interplay between Gary Seven and his super-computer and there are nicely amusing exchanges with secretary Roberta Lincoln (Teri Garr) who has apparently been working for two of Gary Seven’s agents without having the slightest idea that she was working for secret agents from another planet.
There is a bit of a political sub-text but it’s not too intrusive and the main focus is on Kirk’s dilemma. Should he trust Gary Seven or not? If Kirk makes the wrong choice the consequences will be unthinkably horrific. The loneliness of command and the pressures of having to make decisions that could mean life or death for thousands or even in this case millions are recurring themes in Star Trek and these themes propel some of the very best episodes.
This episode works so well because the audience is kept as much in the dark as Kirk - we really don’t know which way he should jump.
There’s a nice mix of humour, mystery and suspense. It all adds up to a very good episode.