In the Star Trek episode, "Tomorrow is Yesterday," the USS Enterprise is thrown back in time to the 20th Century of Earth. The crew saves the life of an Air Force pilot, Captain John Christopher, by beaming him aboard.

Fearing Christopher could disrupt the timeline if returned to Earth after glimpsing the future, Kirk decides Christopher must stay with the Enterprise. After Science Officer Spock later discovers that the pilot's own as-yet-unborn son will play an important role in a future space mission to Saturn, Kirk realizes he must return Christopher to Earth, but without any knowledge of Enterprise or other future events.

When the Enterprise moves forward in time again, it beams Christopher back into his F-104 fighter jet at the very instant after he was beamed off his jet. When Christopher returns to his fighter, he has no memory of being aboard the Enterprise.

Why does he have no memory of events aboard the Enterprise?

This seems like a plot hole to me. (Or a plot device made up by scriptwriters who didn't want an Air Force pilot knowing about the future.)

  • 2
    I think you answer the question as well as is possible with your last paragraph.
    – Politank-Z
    Feb 28, 2017 at 3:48
  • A better explanation IMHO (but one that's inconsistent with what we saw on screen) is that Captain Christopher did remember everything that happened, but chose not to say anything about it. He knew that his son would lead the first Earth-Saturn expedition, he knew that the future looked pretty good, and he didn't want to mess that up. We only saw him for a few seconds after he was beamed back into is plane, and we don't really know from that what he did or didn't remember. May 20, 2018 at 22:00
  • Ultimately despite the writer's intent this was a delete the duplicate. Additionally the writer thinks there can only be exactly one version of something at any point in time despite the time travel. That's why the first version of the Enterprise disappeared from the sky making the episode never happen. By beaming the Capt. Christopher from the ship back to the jet it let's the timeline decide which one to keep. Which is not how time travel generally works nor beaming. Aug 30, 2023 at 23:00

5 Answers 5


From the episode's script, it seems that the writers tried to address this problem, but their explanation is, in my opinion, very unsatisfactory:

CHRISTOPHER: Slingshot effects are fine for you people. How do you propose to return the Sergeant and me?

SPOCK: Logically, as we move faster and faster toward the sun, we'll begin to move backward in time. We'll actually go back beyond yesterday, beyond the point when we first appeared in the sky. Then, breaking free will shoot us forward in time, and we'll transport you back before any of this happened.

KIRK: You won't have anything to remember, because it never would have happened.

No, it doesn't make any sense. But that's the canon explanation: going back in time erased Christopher's memories of everything that happened since the point he was sent back to.

The only piece of in-universe explanation I can speculate on is this: since nobody onboard the Enterprise lost their memories, it must have had something to do with being beamed off of the ship while it was undergoing the slingshot. The transporter bypassed the ship's warp field's temporal normalization actuators, or something like that. Technobabble is the best answer I think we could expect in this instance.

Time travel in Star Trek is hopelessly inconsistent: exhibit A.

  • 4
    You raise some interesting points there by pointing out that the Enterprise crew did not lose their memories. And I can now see another inconsistency. If the ship went even further back in time before it first appeared, then why didn't the Enterprise encounter an earlier version of itself? It's not just time travel in Star Trek that is inconsistent. Almost all treknobabble is hopeless.
    – RichS
    Feb 28, 2017 at 4:54
  • 4
    Just remember: a cannon is a sort of weapon. Feb 28, 2017 at 5:01
  • Also, this seems a rather long range transport.
    – ths
    Feb 28, 2017 at 11:52
  • Ah, Star Trek BS...
    – wogsland
    Feb 28, 2017 at 18:46
  • 3
    You guys are holding a series that used salt and pepper shakers as advanced medical instruments to a pretty high standard. The level of general scientific knowledge, period, as well as its general level of understanding throughout society, let alone pop culture and media is naturally going to be more advanced now, over half a century later. Our understanding of what is and, certainly, what is not possible/plausible is better developed. That they didn't incorporate our standards into their work isn't something to sneer about, necessarily. Feb 28, 2017 at 22:17

The episode's novelisation is similarly unsatisfactory. Interestingly, Kirk spots precisely the same plot-hole that you noticed but accepts Spock's explanation that transporting the pilot back into his ship several minutes before they beamed him out will result in the events never having happened in the first place for him to have a memory of them.

“… and as we mount the other leg of the curve, there will be a slingshot effect that will hurl us forward in time again. If this is most precisely calculated, we will pass within transporter range of the Earth within two or three minutes before the time when we arrived here the first time, before we first appeared in the sky. At that moment, we reinject Captain Christopher into his plane-which will not have been destroyed yet-and the whole chain of consequences will fall apart. Essentially, it will never have happened at all.”

“Are you sure of that?”

Spock raised his eyebrows. “No, sir, of course I am not sure of it. Mr. Scott and I think it may work. The computer concurs. Certainty is not an attainable goal in a problem like this.”

“True enough,” Kirk agreed. “But I don’t see that it solves our problem with Captain Christopher at all. It gets him back home, but with his memories intact-and that’s what we have to avoid at all costs. I would rather destroy the Enterprise than the future.”

There was a brief silence. Both Spock and McCoy knew well what such a decision had cost him. Then Spock said gently, “Captain, Mr. Scott and I see no such necessity. Bear in mind that Captain Christopher will arrive home before he was taken aboard our ship. He will have nothing to remember-because none of it will ever have happened.”

Star Trek 2: James Blish

Personally I lean towards the theory that Spock is lying and is planning to simply dispose of the pilot (who at this point is a time-travel duplicate from an obsolete timeline) by just beaming him into the nearest volcano.

  • 3
    Excellent point in your last paragraph. I'd argue that deleting the duplicate's experiences is tantamount to killing him anyway. Volcano does seem to be the most logical option. Feb 28, 2017 at 21:53
  • @Valorum I prefer into the sun myself, less margin for error
    – Steve
    Jun 6, 2018 at 17:50

It's one of actually two plot holes caused by the beaming.

Even if Spock and Kirk's handwaving about "it never would have happened" somehow magically erased Capt. Christopher's memory, there still is the reason why he needed to be beamed out in the first place. Remember, the earlier Enterprise had locked onto his fighter with a tractor beam, which was causing the fighter to break apart.

Christopher is being beamed back into a disintegrating aircraft. He will die within seconds and the timeline is once again disrupted. The issue of his memory is moot.

What follows is an alternative ending to the episode that would have solved both plot holes. If it bothers you that the ending below is not canon, then vote based on what I said above (which is canon).

The time is immediately before Christopher's first encounter with the Enterprise. There are two copies of the Enterprise present. Let's call the copy that is damaged and foundering in the atmosphere E1. The second copy -- call it E2 -- has Christopher and the security guard on board, has been repaired, went back again in time to meet E1, and is at a higher altitude.

The solution is for E2 to use its tractor beam to pull E1 higher into the atmosphere.

Both ships disappear from Air Force radar. Christopher never spots the Enterprise. E1 has no need to use its tractor beam on Christopher's fighter. Christopher and his fighter are completely unaffected by the incident. Earth's history goes on as normal.

With the normal Earth timeline restored, Christopher and the security guard disappear from E2. Optionally (it does not matter) the crew of E2 are unaware that they even had passengers! E2 hails E1 and instructs them that -- after completing repairs -- they need to go back in time to become E2. The Enterprise's timeline is now complete.


There is an article on Star Trek time travel in *the Best of Trek # 2 * in which it is concluded that it is impossible to know what happened to Captain Christopher and the guard. The article says that beaming human bodies into spaces already occupied by human bodies would be deadly, for reasons that should be obvious.

In Menagerie part 2":

NUMBER ONE: Now, you all know the situation. We're hoping to transport down inside the Talosian community. SPOCK: If our measurements and readings are an illusion also, one could find oneself materialised inside solid rock.

In "Return to Tomorrow":

SCOTT: I don't like it, sir. The transporter co-ordinates preset by an alien of some sort. You could materialise inside solid rock. MCCOY: Inside solid rock? SPOCK: Unlikely. These co-ordinates correspond with the location of the subterranean chamber.

In "Tomorrow is Yesterday" our heroes say they are going to beam two persons into the the space occupied by their past bodies - which should be just as deadly as beaming into solid rock - and do seem to beam those two persons SOMEWHERE. And yet the bodies they are allegedly beamed into don't turn into horrors out of the infamous Star Trek: the Motion Picture transporter sequence.

Thus something else, somewhere on the spectrum from wonderful to horrifying, must have happened to Captain Christopher and the air force guard.

There is no evidence that Kirk, Spock, & co. planned to dispose of their guests and were lying about what they planned to do. But their plan made no sense. Thus they seem to have been under some influence that made them think a senseless plan would work, and under that influence they seemed to beam their guests SOMEWHERE.

For the sake of Captain Christopher and the Air Force Guard, we can only hope that mysterious unknown mental influence had a decent fate planned for them.

Thus the question of why Captain Christopher forgot everything he experienced aboard the Enterprise is based on the false presumption that the future Captain Christopher was beamed into the space occupied by the past Captain Christopher. That could not possibly have resulted in both or either of his selves surviving - and we did see one of them surviving the experience.


It's a plot hole, poorly handwaved in dialog. The Enterprise going a little further back in time would have meant that there were briefly two Enterprises when Christopher flew his intercept. I think what the writers were trying to suggest is that the intentional slingshot was supposed to somehow erase the timeline which existed from the point the Enterprise first arrived in the 1960s and allow the crew to replace it with a reconstruction of the original timeline.

By the way, the plane Christopher was flying was an F-104 Starfighter. This was an interceptor designed to attack enemy aircraft. As such, its typical armament was canon and air-to-air missile (conventional explosives). Although later models could carry a nuclear weapon, such a device would not be for air-to-air use - would have been a bomb, not a missile, so would have been no threat to Enterprise.

  • Except for the temporarily disabled state of enterprise when shields were down. This caused the timeline issues and allowed the Enterprise to beam the 20th century people aboard
    – Steve
    Jun 6, 2018 at 18:02

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