All Our Yesterdays is a third season episode of Star Trek: The Original Series, first broadcast March 14, 1969. It is episode #78, written by Jean Lissette Aroeste , and directed by Marvin Chomsky . It guest starts Mariette Hartley as Zarabeth.

Quick Overview: The crew of the Enterprise are trapped inside a time portal on a world threatened by a supernova.

On stardate 5943.7, the USS Enterprise arrives at the planet Sarpeidon to evacuate inhabitants that are doomed by an impending supernova explosion of the system's star. Oddly, sensors detect few signs of humanoid life on the surface and Captain Kirk, along with Dr. McCoy and Mr. Spock, beam down to investigate.

Once there, the landing party locates a sophisticated computer library where they encounter a man named Atoz. Atoz explains that the library contains an archive of historical data disks. Kirk warns Atoz of the planet's imminent destruction and that they must leave immediately. Atoz however, says he is aware of the destruction of his world, and is already prepared. He informs that he will be joining his wife and family soon.

He then shows them a machine called the "Atavachron", and by inserting the historic disks into the device, the machine opens a portal that can send any who enter back in time to the point recorded in the data. Atoz informs them that all of the planet's inhabitants have escaped their demise by going through the portal to their past, which explains why only Atoz is left. He was just about to depart when the landing party arrived.

Atoz activates the machine and gets ready to enter, but and Kirk hears a woman's scream on the other side of the portal. Without thinking, he rushes through and disappears. McCoy and Spock rush into the portal after him despite Atoz's warnings not to go through since they have not been "prepared". The two find themselves transported through time 5000 years in the past to when Sarpeidon was in the midst of an ice age. The two are trapped in the brutal cold and desperately look around for their captain.

Meanwhile, Kirk arrives in an alleyway at a point in Sarpeidon's history reminiscent of Earth's Victorian Era. He manages to rescue a deshevled looking woman from being stabbed by a knife wielding attacker. Kirk chases the assailant away, but then discovers the "maiden in distress" is really a thief that was attempting to pickpocket the man.

Kirk returns to the alley but cannot locate the time portal. He discovers he can still talk to McCoy and Spock but cannot get to them. Authorities arrive and arrest Kirk for helping the thief. At the same time, McCoy asks his captain about what is going on at his end, and the guards are shaken when they hear the mysterious disembodied voice. The thief then betrays Kirk and tells the guards that he is using witchcraft, and the voices forced her to steal against her will.

On the verge of freezing to death, Spock and Bones look for shelter and encounter a beautiful woman wrapped in heavy furs. She identifies herself as Zarabeth, and kindly leads the strangers to a warm cave. Zarabeth explains that her kinsmen have been exiled when an attempt was made to kill their tyrannical leader Zorkon.

Meanwhile, Kirk is taken to a jail cell and interrogated by a prosecutor. Kirk mentions the "library" where he came through time, and the prosecutor becomes nervous. At first, the prosecutor believes that Kirk is innocent but the guard and thief maintain they heard the mysterious voices and that Kirk must be a witch. Kirk knows the prosecutor is hiding something, and presses the fact that he came through time. The prosecutor gets angry and rushes away, saying he wants nothing more to do with the prisoner.

Back in the ice age, Zarabeth listens to McCoy and Spock's story of their arrival, and informs them that she too is from Sarpeidon's future. She explains that the Atavachron portal is "one-way" and alters a traveler's molecular structure so they cannot return back to their time or they will die.

Meanwhile, Kirk sits in his cell and overpowers the guard when he brings him food. The prosecutor arrives and Kirk confronts him, accusing him of being from Sarpeidon's future and threatening to expose him to the others as a "witch" if he does not help him escape. The prosecutor tries to calm Kirk down, admitting that he is indeed from the future but Kirk cannot return. Kirk thinks otherwise, and forces the man to help him locate the time portal. The two sneak out of the jail and head to the alley where they eventually find the doorway. The man warns of the effects of time travel without preparation and that Kirk cannot return or risk death.

Kirk ignores the warning and steps through the portal returning to the library. He confronts Atoz and demands he help him rescue his friends. Atoz refuses to help until Kirk "prepares" himself before returning to the past. Believing his friends are dying and no more time can be wasted, Kirk tries to force Atoz to cooperate, but he zaps Kirk with a weapon, knocking him unconscious.

Back in the ice age, Mr. Spock slowly reverts to the barbaric nature of ancient Vulcans; an effect, Zarabeth claims, that is the result of remaining too long in the past. McCoy notices the hostile changes in Spock when the Vulcan reacts angrily to one of his typical insults. Spock begins to fall in love with Zarabeth, losing all intention of returning to the future anyway and wants only to remain with his new love. McCoy however, is not convinced that they are trapped, and believes that only Zarabeth is somehow unable to return.

Back in the library, Kirk awakens on a cart just in time to stop Atoz from wheeling him into the time portal, and forces him to find his companions. Trying various disks, Kirk locates Spock and McCoy in the ice age and opens a portal. Meanwhile, McCoy hears Kirk's voice and manages to drag Spock away from Zarabeth long enough to follow it back to the library. The three are happily reunited, and with time running out, they watch Atoz slam a disk into the machine and make a panicked run for the portal and disappear.

Realizing they have seconds before the planet is destroyed, the landing party quickly return to the Enterprise which safely warps away just as the star explodes.

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