The Enterprise accidentally travels back in time to Earth in the 1960s, where it’s seen as a UFO, and then struggles to erase records of its visit and to return to its own time. 
- This is a fun episode that doesn’t bear close examination of its plot points or theoretical basis.
- The episode opens, as “The Conscience of the King” did, with a brief scene without reference to the Enterprise – in this case, a contemporary US Air Force base reacting to the appearance of a UFO, and scrambling a jet to approach it. (No date is mentioned, but it’s presumed it’s contemporaneous with the viewer’s present, in early 1967.)
- And then it’s quite striking to see that the UFO is – the Enterprise! One of the best episode teasers ever.
- Despite Memory Alpha (http://memory-alpha.wikia.com/wiki/Tomorrow_is_Yesterday_(episode)), Kirk does *not* say ‘subjective time’ in his log entries. After all, in the first one, he doesn’t yet realize the ship has been thrown back in time. It does however raise the question of what ‘stardate’ actually means, but this is a topic that, though raised many times, has no satisfactory resolution.
- There are two areas of mind-boggling coincidence in this story. The first is the sheer physical exactitude with which the Enterprise, having encountered a “black star” and then been thrown some light-years, presumably, off its course, should happen to land *just within the extremely thin atmosphere* of Earth – not smashing into the planet, not arriving at some random point in space billions of miles away – and furthermore, not still moving at some great speed — but rather, as we see, limping slowly upward into Earth’s sky. (The image above is from the original show; the enhanced graphics on the currently available Blu-Ray discs greatly improve the images of the Enterprise among the clouds.) And over the US, another coincidence!
- Later, at the end of the episode, similar miraculous exactitudes are achieved when the Enterprise, having slung-shot around the sun to accelerate and reproduce the time effect (though for some reason this time in the other direction, toward the future), passes quickly by the Earth and while doing so manages to time the beaming down of both its accidental tourists from this era who’ve managed to get aboard the Enterprise—to exactly the spots, or close enough, from where they were first beamed up. And via a chain of vocal commands! To achieve such precision is, to put it mildly, amazing.
- A nit: Captain Christopher is beamed out of his jet in flight position, yet appears on the transporter pad standing up.
- Christopher, 1960s US Air Force pilot, is appropriately amazed by appearing aboard the Enterprise, as he gradually gathers what kind of ship it is. Cushman suggests this is part of the charm of this episode – it visualizes a viewer fantasy for a contemporary person to actually *be* on the Enterprise.
- A female crewman? Wow. Kirk mentions that there are only 12 like this ship in the fleet. It was an accident that the Enterprise is here; Christopher replies, “You seem to have a lot of them,” referring to the prevalent speculations about UFOs at the time.
- Spock quickly realizes the consequences of Kirk’s rather rash decision to beam Christopher aboard: now that Christopher has seen the future, he cannot be returned to Earth of this time, lest he use that knowledge to make certain speculations that might change history – and wipe the Enterprise and all aboard out of existence, like a soap bubble. There are of course many, many theories of time travel among science fiction stories, of which Spock’s worry is a common one, but far from the only one; there are many other possibilities. (Examples: the past is fixed and cannot be changed, so that Christopher’s efforts were he returned would have no effect; or, the past might be changed and branch a separate timeline.)
- This episode features a gratuitous humor element that I’ve never cozied to: the computer Kirk speaks to replies in a sultry, female voice, and calls him “dear”. Cpt. Christopher, hearing this in Kirk’s cabin, observes how people of his era have such interesting problems. Spock drolly explains that the computer system was programmed by the female society on Cygnet XIV, which gave it, of course, a female personality. And that to fix it would require a complete three-week overhaul at a starbase. But really: three weeks?? Whereas now we can change the voice option on our phone or in our car with a few swipes, in a few seconds. Failure of imagination.
- Anyway, the voice is sexist. But fans at the time – and the producers, and NBC – loved it.
- Act 1 ends as Scotty repairs the engines, but observes: “We have no place to go in this time!”. A startling observation, but… how could this be true? Did the entire Federation of Planets, as we later understand it, arise only because *Earth* emerged into the galaxy? Weren’t there other races who’d developed space flight and colonized other worlds before humans, whom the Enterprise might have sought out in Earth’s 1960s? Surely one would think so, but Trek producers and writers never thought very deeply in these directions, or avoiding doing so to not complicate scripts.
- In a sloppy bit of plotting, as in “The Alternative Factor”, Cpt. Christopher is allowed to wander around the Enterprise decks on his own. Allowing him to knock out a crewman, steal his phaser, and use it to try ordering the transporter room guy to beam him down. Kirk shows up to belt him.
- Leading to the scene in which Spock reveals that Captain Christopher’s *son* — not yet born! – will have an effect on history, heading the first successful Earth-Saturn probe. Christopher says he doesn’t have a son. McCoy, with a twinkle in his eye, says “You mean yet!” And Christopher realizes what this means. “I’m going to have a son…!” It’s a legitimately charming moment.
- Then we have Kirk and Sulu beaming down to the base to try to retrieve whatever camera recordings and other data that would leave evidence of the Enterprise’s visit. It’s always struck me as amazingly coincidental that, by opening up a computer unit with tape reels, they can identify which reels contain evidence of their visit; and a bit later, find the photographic evidence too, so easily.
- This episode is regarded as the first deliberately semi-comedic episode – the result of D.C. Fontana’s script. The next scene (after the female computer voice) that displays this is Kirk’s being captured and playing dumb. “I popped in out of thin air.” He’ll be locked up for 200 years. “That ought to be just about right.”
- The episode’s endgame has the Enterprise trying to recreate the slingshot/time travel effect by swinging around Earth’s sun (though, as above, they think it will throw them forward in time, not back). In the original episode, there were no special effects to support this – we never saw the sun itself, just the Enterprise jiggling in space. The enhanced graphics for this episode do show the sun, and in fact shows the Enterprise banking closely around the sun – but in a nonsensical way that echoes the way the Enterprise has always been shown to ‘bank’ around planets it orbits – visibly arcing, and banking like a jet fighter would do in an atmosphere.
- A recurrent Trek bad physics theme: “Since we passed Mercury… the sun’s pull has increased on us greatly.” No. The Enterprise is flying though space; the “sun’s pull” is only a matter of being in orbit, or doing a flyby; it’s a matter of the trajectory the Enterprise will take around the sun. It’s not a matter of the sun *pulling* on the Enterprise, as if it were an object not moving relative to the sun.
- Anyway, this sun flyby works, as the Enterprise accelerates through the warp drive, and then passes Earth and achieves those amazing precise beam downs….
- The Enterprise “passed Pluto”. Well, no; this presumes that the planets around our sun are somehow lined up, so that the Enterprise entering the solar system, or leaving it, would pass by the planets in order. While of course, at any given moment, the planets are in their orbits but scattered around the sun in their orbital arcs. Would have been better to say, “passed Pluto’s orbit.”
- Of course, the Enterprise might well have been heading somewhere in the galaxy not along the Sun’s ecliptic; that is, at some acute or right angle to the plane of the planets. So ‘passing Pluto’ or ‘passing Pluto’s orbit’ would be an academic point, since the ship might well be headed in some relatively perpendicular direction. Trek routinely assumed – to the point of visualizing the Enterprise’s encounter with other spaceships, and the way it departed planets at the end of episodes – that the galaxy lay in some flat plane. Yet even the Milky Way galaxy’s plane doesn’t match the solar system’s. A prominent Trek example of how complicated physics and astrophysics is simplified for intuitive, non-scientific viewers.
- And finally, the story ends as the Enterprise winds down its warp-drive thrust into the future… and gets voice confirmation from Starfleet that they are back in their age. Really? Exactly back in their age? The exact time they left from, before encountering that ‘black star’? Don’t ask.
- Again, a fun episode that does not bear too close examination.
- Music notes: the whirling music as the Bluejay 4 pursues the Enterprise is the ‘spinning cube’ music from The Corbomite Maneuver; and in the next scene, Cpt. Christopher’s view of the Enterprise is underscored with the ‘Fesarius’ theme from that same episode score.
- The music as Kirk and Sulu slowly wander the hallways of the air force base is the slow, creepy version of the main Mudd’s women theme. We hear the “Shore Leave” bunny music as a wandering security policeman is beamed up.
- The fight in the photo lab is scored with Finnegan’s fight theme from “Shore Leave”.
- When the security policeman gets his chicken soup in the transporter room, we hear a version of the “Shore Leave” theme; the following scene, back at the base, opens with Courage’s “No Man” theme.
Blish’s adaptation, in ST2:
- Blish omits the initial off-ship scenes of the US air base detecting the ‘UFO’ and sending up a fighter to bring it down. But he does spend a couple long paragraphs at the beginning providing some astrophysical background about the notion of “black star.”
- In passing, Blish establishes that warp 4 is 64 times the speed of light.
- On the one hand Blish describes the Enterprise as being in a “fixed orbit” around Earth, but on the other hand that the ship is “too low in the atmosphere to retain this altitude,” which doesn’t sound like an orbit at all. So even Blish doesn’t seem to have a reasonable understanding of orbits or how things move through space.
- As Blish does several times in his books, he describes Uhura as a “beautiful Bantu girl,” which surely was never in any script.
- In the scene in which Spock relates his research in the future contributions of this Air Force pilot, Cpt. John Christopher, Blish (unlike the scriptwriters) has Spock note that “There was a popular author by that name, but it was a pen name; you are not he.” He’s referring to http://www.sf-encyclopedia.com/entry/christopher_john.
- Similarly, Blish adds a bit of background or perspective that didn’t occur to the scripters. When Scott says the engines are working, but “we’ve no place to go in this era,” Blish has him go on: “Mister Spock tells me that in the 1970s the human race was wholly confined to the Earth. Space outside the local group of stars was wholly dominated by the Vegan Tyranny, and you’ll recall what happened when we first hit them.” Fascinating.
- And Kirk clarifies, for Christopher: “There is no such solar planet as Vulcan… Mr. Spock’s father was a native of The Vulcan, which is a planet of 40 Eridani.” 40 Eridani being a real star, only about 16 light years from the sun (https://en.wikipedia.org/wiki/40_Eridani). But no Trek episode ever established Spock’s Vulcan as being a planet of 40 Eridani. Anyway Spock warns: “If we took the Enterprise there, we would unwrite their future history too.”
- Blish omits the entire subplot of Kirk and Sulu beaming down to retrieve Air Force evidence of the Enterprise, their capture by a policeman, and the inadvertent beaming up of yet another local.
- As they discuss the idea of slingshotting back into the future, Blish has Kirk made an extraordinary concession: “I would rather destroy the Enterprise than the future.”
- And as Christopher expresses distress for possibly losing his memories of this glimpse of humanity’s future, McCoy offers some advice, which at first repulses Christopher, then deeply impresses him. McCoy: “In perhaps sixty more years, or a few more, you will forget things many times more important to you than this—your wife, your children… You will forget every single thing you ever loved, and what is worse, you will not even care.” Christopher is shocked; but McCoy goes on: “I’m only trying to remind you that regardless of our achievements, we all at last go down into the dark. …I’m trying to call to your attention the things that are much more valuable to you than the fact that you’ve seen men from the future and a bucketful of gadgetry. You will have those still…” He goes on; it’s quite a passage.
- Blish, more than once, refers to a “navigation tank” located presumably at the helm and nav station – a suggestion that a depiction of 3-dimensional space would need a 3-dimensional image, not a flat, round panel.
- And Blish implicitly acknowledges the absurdity of the crew manually timing the beam down of Christopher as the Enterprise flings past Earth: “This was going to have to be the most split-second of all Transporter shots. No human operator could hope to bring it off; the actual shift would be under control of the computer.”
- And Blish avoids the question of whether the Enterprise returns precisely to its own time, by not mentioning it at all, nor having the Enterprise contact star fleet command. Instead Blish ends with Spock alluding to the poet Omar, and the passage about the moving finger that, having writ, moves on.
