"Tomorrow is Yesterday" may be the best time travel episode in Trek though such travel is easily the worst part of it. The beginning is certainly exciting, as a F104 is hastily scrambled to intercept a "real UFO" that just appears in the late 1960's sky, as NCC-1701 has been thrown back in time after a close encounter with a "black star." (Time travel stories are inherent to the nature of warp travel yet somehow that dish tastes better without knowing how it is made.) Had TOS as originally intended used this episode to wrap up a "Naked Time" cliffhanger it might have reduced the temptation to revisit the idea again and again with deteriorating results every time.
Anyway, damaged and unable to outrun their pursuer Enterprise catches it instead in a tractor beam that soon crushes the fragile jet. Its pilot is rescued at the last second, presenting a problem about how to return him without distorting the "known" timeline. Kirk and Sulu break into an airbase to recover wing camera footage and they inadvertently pick up another unexpected guest. To solve their predicament Enterprise sets up a slingshot pass at the sun, emerging to materialize her passengers at exactly the moment from which they were snatched before (using a miraculous property of the transporter to rearrange posture from sitting to standing and crouching to walking!).
Much of the drama revolves around pilot Captain John Christopher and actor Roger Perry does an excellent job generating sympathy while handling the situation from his character's unwavering perspective. (Of course being encouraged by the thought of yet unborn son, pioneer on the first manned mission to Saturn, doesn't hurt!) Fellow guest star Ed Peck does a convincing job as the frustrated Air Force interrogator Lt. Col. Fellini, playing inadvertent foil to Kirk's bemused recalcitrance ("Believe me, you wouldn't believe me"). Even Hal Lynch is memorable as Air Police Sergeant Chicken Soup.
Like most D.C. Fontana stories the crew is in helpful form here with the exception of a computer reprogrammed by women getting annoyingly affectionate. Spock and McCoy serve to complement the concerns both about keeping Captain Christopher and returning him (at least while keeping him remains an option). Unlike other "take me with you" type stories Captain Christopher never abandons a grounding in his own timeframe although he is certainly grateful for the experience. In a way he represents the viewer, and his sign-off to Kirk represents a position held by many regarding the series altogether: "Thanks for the look ahead."