"The Alternative Factor" is a first season episode of the original science fiction television series, Star Trek. It is episode #27, production #20, and was broadcast on March 30, 1967. The episode was written by Don Ingalls, and directed by Gerd Oswald.

In this episode, the crew of the USS Enterprise encounters a "reality jumping" madman. This is the first Star Trek episode to deal with a parallel universe.

The Enterprise, completing a mapping assignment, is rocked by an energy pulse. Science Officer Spock informs Kirk that the gravity pull of the planet fluctuated to zero and says that the surrounding space seemed to momentarily "wink" out of existence.

Sensors locate a human presence on the planet that wasn't there before. Mr. Spock and Kirk beam down to the planet and find a one-man spacecraft. A disheveled, bearded man appears and accidentally slips off a cliff. The man survives the fall but is injured, and Kirk has him beamed to the Enterprise for examination.

Back on the ship, Lt. Masters informs Captain Kirk that the mysterious disturbance has drained the dilithium crystals in the warp drive. A message from Starfleet reports that every quadrant has been subjected to the same winking effect and electronic disruption as the Enterprise. Starfleet fears that the disruption may be a prelude to an invasion and has ordered all ships except the Enterprise to leave the area. Kirk is ordered to find the cause of the disturbance, alone.

Lazarus frequently fades in and out of the universe, encountering his nemesis in a dimensional corridor, creating an energy wink, rippling through the universe. Spock reports a "rip" in space and time on the planet. Lazarus insists that his enemy, trying to destroy the universe, is causing the phenomenon. Lazarus demands dilithium crystals so he may fix his ship and continue to fight his enemy. Kirk refuses; Lazarus steals dilithium from the Enterprise and is caught. Lazarus denies the theft and blames the theft on his yet unseen nemesis.

Kirk beams back to the planet with Lazarus and a security team to seek this "hidden" enemy. Lazarus has another "dimensional corridor" episode and is returned to sickbay. Lazarus explains to Kirk that he is a time traveler; the planet below was once his home world. Lazarus claims his nemesis destroyed his civilization in the past, for which Lazarus has chased him for centuries.

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