This early D. C. Fontana episode bravely goes where TOS never went before - to the time of its own creation - the late 1960s. Another bold frontier for the series (previously explored, but less interestingly, in Squire of Gothos) was crossed as Tomorrow is Yesterday established the show's sense of humor by - characteristically - poking fun at its own historical context. The original series was never equaled in any of its worthy successors in making commentary - whether comedic or serious - on its own historical context. And though not terribly serious, Tomorrow is Yesterday is a good example.<br /><br />The plot is fairly complicated and as impossible (from a scientific perspective) as most of what passed for science in Voyager and Enterprise, but the strength of Fontana's dialog and the performances of the regular crew and the guest stars (Roger Perry is great in this) carries it. Enterprise, on its way to earth, is catapulted back in time by a slingshot effect relating to the gravitational pull of a star, to the late 1960s (ya, I know, a physics class might have helped here, but bear with me). The ship enters earth's gravitational field partially disabled and in a decaying orbit. They are spotted and branded a UFO, and a warplane is sent to investigate. The crew must figure out a way to repair the time line, return some unwanted guests to earth without memories of visiting the Enterprise, and, of course, return to the 23rd century without ripping the ship apart.<br /><br />The episode is well written, despite the ridiculous premise, and very nicely acted. The cinematography is effective, though the effects are some of the worst in the series, and many of the long range shots of the ship should probably have been cut. Kirk engages in one of his better fight scenes - if you're into that sort of thing.<br /><br />An entertaining episode, and one that established a trope for the entire franchise - engagement with the show's own historical context. Recommended.