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Tomorrow Is Yesterday
Star Trek Original series, season 1, episode 19

Originally aired Jan. 26, 1967
Writer D. C. Fontana
Director Michael O'Herlihy
Guest cast Majel Barrett (computer voice)
Marc Dempsey (Air Force captain)
Hal Lynch (Air Police sergeant)
Richard Merrifield (Webb)
Ed Peck (Colonel Fellini)
Roger Perry (Captain John Christopher)
Jim Spencer (air policeman)
Sherri Townsend (crewman)
John Winston (Transporter Chief)
F-104 Starfighter in Star Trek Tomorrow Is Yesterday
Captains Kirk and Christopher in Star Trek Tomorrow Is Yesterday
Scotty struggles to keep his feet during the breakaway sequence in Star Trek Tomorrow Is Yesterday
Stardate: 3113.2

Plot summary

The Enterprise is en route to Star Base 9, in the general direction of Earth, when it comes under the gravitational influence of a "black star". In conventional astronomy, two types of stellar object carry the description "black" – black dwarfs and (stellar) black holes. The implication that the Enterprise fell into a steep, powerful gravitational field suggests that the object in question is a black hole. In its efforts to break free, the Enterprise is thrown across space and time. The ship is flung to a location in Earth's atmosphere above the American midwest, forcing the crew to climb to orbital height, in the "late 1960s" just before the first manned moon shot (see Apollo 11, which launched on Jul. 16, 1969).

The opening sequence shows F-104 Starfighter jets being scrambled to intercept what has been detected as a UFO on military radar. Captain John Christopher is the pilot of the plane that, while climbing steeply to high altitude, spots and reports on the intruder. He is ordered to apprehend the enemy vessel, which he realizes is of enormous size and unknown design. Before Christopher can fire his weapons, Kirk orders his plane to be placed in a tractor beam. However, when it becomes clear that the plane cannot withstand the stress of the tractor and is about to break up, Kirk orders the pilot to be brought aboard the Enterprise by transporter.

Now Kirk is faced with two huge problems – returning his ship and crew to its proper time and deciding what to do with Captain Christopher, who has been given a glimpse of the future. This, in fact, is the first episode of Star Trek to deal with conundrums of time travel. As Spock points out, if Christopher is sent back with his memories intact he may be instrumental in changing "what must be". The whole of future time may be rewritten by actions triggered by Christopher's knowledge. Upon hearing of Kirk's dilemma, Christopher comments, "You're as much a prisoner in time as I am."

Then Spock discovers from computer records that Christopher's son, who is not yet born, will head the first human crew to Saturn. This means that the pilot must be sent home, so that he can father the man who will play such a key role in human history. But how can this be done without Christopher contaminating the time-line with his foreknowledge? The first step must be to eradicate the data collected by the military of the encounter with Enterprise. Christopher gives Kirk and his staff information to help them gain access to the records section of his base. Kirk and Sulu beam down and soon find themselves outside an office labeled"Statistical Services Division, 498th Airbase Group (of Air Defense Command). They are apprehended by an air police sergeant who confiscates Kirk's communicator. When Spock tries to contact Kirk, the sergeant accidentally sets off the emergency signal and is beamed aboard the Enterprise. Now Kirk has two potential human contaminants to deal with.

Kirk and Sulu continue their search for evidence of their intrusion and Sulu successfully beams up with the incriminating computer tape and photos. Kirk, however, is arrested and questioned. "I'm going to lock you up for 200 years," his inquisitor threatens. "That ought to be just about right," replies Kirk.

Spock and Christopher beam down to rescue Kirk but at the last minute Christopher gets hold of a gun and, thinking of his unborn son, insists that he be left behind. Spock, having anticipated that he would make such an attempt, comes upon him from behind and renders him unconscious with a Vulcan neck pinch. They all return to the Enterprise, where Spock and Mr. Scott tell Kirk of their plan to return the ship to the 23rd century. The ship was brought to Earth in the first place by a "whiplash" caused by the Enterprise attempting to break free of the dark star's gravity. Now, they explain, they must use the same slingshot effect to get back to their own time. This will involve flying toward the Sun, "seeking out its magnetic attraction", and then suddenly pulling away. Time will reverse as the ship races toward the Sun, according to Spock's theory, then quickly advance as the ship pulls away. Scott is worried about accurate control of the breaking maneuver, which is crucial if they're to arrive back when and where they came from.

As the Enterprise hurtles toward the Sun, the ship's chronometer goes into reverse and Captain Christopher and the air police sergeant are beamed out at the precise times to return them to their original states, just before they found substantive evidence of the Enterprise's intrusion. Thus the time-line is preserved. The Enterprise then breaks away and travels forward in time to its own era.

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