Tomorrow Is Yesterday has a wonderfully disorienting pre-credits teaser. Most episodes to date have opened with a shot of the Enterprise orbiting the planet of the week. But today we begin on twentieth century Earth, which means that for a few moments it’s possible to believe that the wrong channel has been selected and this isn’t Star Trek  after all …

Normal service is resumed after we see the Enterprise (a little wobbly, it’s true) moving through the sky. But an intriguing mystery has been established. How and why have they travelled back through time?

There’s a mundane reason for this atypical opening (the episode was originally planned as the second of a two-parter, following The Naked Time) but no matter, it still works.

The reason why the Enterprise has taken a trip back to the 1960’s won’t detain us for too long (something about the effects of tangling with a high gravity black star).  It makes little sense, but then neither does the way they return to their own time. A little more on that later.

Briefly the episode has a serious and downbeat tone. Captain John Christopher (Roger Perry) is a US pilot sent to investigate a mysterious UFO (which of course is the Enterprise). Beamed aboard, he’s aghast to be told by Kirk that they can’t return him  – he’s glimpsed the future and so could cause untold damage to the timelines if he went back.

There was scope for an interesting story to be crafted out of this dilemma – how Christopher would have adjusted to life in the future – but this wasn’t that episode. Spock quickly realises that they have to send him back (Christopher’s unborn son will become an important figure in the development of space travel) and so the romp begins.

It’s easy to see how the basic plot was recycled for Star Trek IV: The Voyage Home. There’s no whales, but the concept of Kirk and co carrying out a secret mission on contemporary Earth, bamboozling hapless officials as they go, feels very similar.

Kirk and Sulu beam down to the airbase, intent on removing all photographic evidence of the Enterprise’s existence. Kirk gets himself captured, but not before William Shatner has indulged in a fight with several soldiers that’s played purely for laughs. His later interrogation is pretty chuckleworthy too (especially the comment about his uniform). Shatner’s clearly having a ball in these scenes.

If it wasn’t obvious that Tomorrow Is Yesterday was the series’ most comic offering to date, then the soundtrack tends to hammer this point home at every available opportunity. For example, whenever Kirk talks to the computer (which now has a female personality and is deeply in love with him). Yes, it’s that sort of story.

If the reason for the Enterprise’s arrival in the twentieth century lacks a little logic, then that’s nothing compared to the plan cooked up by Spock and Scotty to get them back to where they belong. Best not to worry about this though, just go with the flow. The reset button solution does solve the problem of Christopher (not to mention another hapless individual who also got beamed up) but it’s also a massive contrivance.

Mind you, there are some episodes where I find plot-holes to be annoying and others (like this one) where I’m inclined to be more forgiving. Tomorrow Is Yesterday is frothy, uncomplicated fun and therefore impossible not to enjoy.