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"The Alternative Factor" was the 20th episode of Star Trek: The Original Series, in the show's first season, first aired on 30 March 1967. The episode was written by Don Ingalls, directed by Gerd Oswald and novelized in Star Trek 10 by James Blish.

Investigating a "winking out" of the universe, the Enterprise finds a time traveler named Lazarus who has counterparts in both the positive and antimatter universes. If both versions of Lazarus meet at the same time—outside of a magnetic corridor between universes—it will mean the end of everything!


The USS Enterprise.

The USS Enterprise is preparing to get underway, finishing a planetary survey in standard orbit around an iron-silica-type uncharted planet. Suddenly, the starship is struck by a strange effect, being buffeted by a strange magnetic "blinking" effect where the surrounding universe, to the edge of sensor range, seems to partially fade out of existence. After the effect ceases, scanners show a humanoid lifeform on the planet surface.

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Log entries

Captain's log, stardate 3087.6
While investigating an uncharted planet, the Enterprise, and at least this entire quadrant of space, has been subjected to violent, unexplained stress and force. Sensors have reported the presence of a Human being on the planet below who might be connected with the phenomenon. With my first officer and a security team, I have set out in search of him.


Captain's log, stardate 3088.3
We continue to orbit the dead planet, which seems to be the source of the phenomenon which has struck the Enterprise, and all sections of the galaxy, once again. As for Lazarus, the story he tells me about the humanoid continues to trouble me.

The USS Enterprise.

Captain's log, stardate 3088.7
We are no closer to finding an answer to the strange phenomenon than we were at the beginning. Not only have two of my crewmen been attacked, two of our dilithium crystals are missing, and without them, the Enterprise cannot operate at full power. They must be found.



Episode characters

BarstowClifford Brent[1]Bill HadleyHarrison (Lieutenant)James T. KirkLazarusRyan LeslieCharlene MastersLeonard McCoyOlivia RoseSpockNyota Uhuraunnamed 2260s USS Enterprise personnel (USS Enterprise personnel)

Novelization characters

BarstowJames T. KirkLazarusRyan LeslieCharlene MastersLeonard McCoySpockNyota Uhura

Starships and vehicles

USS Enterprise (Constitution-class heavy cruiser) • Lazarus' spaceship


Lazarus's planet
Referenced only
EarthStarbase 200

Shipboard areas

USS Enterprise
bridgebriefing roomcartographic section (referenced)dilithium reamplificationcorridorengineeringmain engineeringquarterssickbay

Races and cultures


Technology and weapons


States and organizations

FederationStarfleet (Starfleet Commandcommand divisionoperations divisionsciences division)

Technology and weapons

computer bankexperimentation chamberexplosivesmagnetic communication satellitephaserphaser banksafety valvesensorsspaceshipstarshipviewscreen

Materials and substances

antimatteratmospherecoffeebloodcrystaldilithiumdilithium crystalhydrogenirongasoxygensilicon

Ranks and titles

captaincommandercommodorecommanding officercommunications officercrewmandoctorengineerFederation Starfleet ranksFederation Starfleet ranks (2260s)first officerlieutenantlieutenant commandermedicofficerphysicianscientistsecuritysecurity teamscience officer

Other references

abrasionalternative warpanatomyantimatter universearmbandagebattle stationsbeamblindness • "Bones" • bootbruisebullcaptain's logclothingCode Factor 1deafdevildilithium reamplificationdimensiondimensional doordinosaurdizzinesselectromagnetismenergizer/energizing circuitsenergyFahrenheitFederation membersfive-year missionfootprintforeheadgalaxygeneral alertgovernmentgravimetric fieldheavenHellhomeworldhumanoidlanding partylanguagelife surveylifeformlog entrylogicmagnetic corridormagnetic effectmagnetic fieldmagnetismmattermedicinememorymultiversemusclemannation-statenegative magnetic corridororbitouncepantsparallel universeparanoiaparsecphotographic sectionplanetpoetrypowder kegpositive universepriority oneprisonpulsation phenomenonquadrantraces and culturesradiationrankrecuperative powersred 2 messagerip in the universesciencespacestandard orbitstarstar chartstardateStarfleet General Orders and RegulationsStarfleet uniformStarfleet uniform (2260s)technologytimetime chambertime shiptime traveltitletunicuniformuniversewaterweaponzero gravity


prior to the discovery of Lazarus
Lazarus crosses over and encounters his double. (prior to episode/novelization)
stardate 3087.6, 2267 (2260s chronology, 2264–2270 Enterprise voyages)
Enterprise travels to Lazarus's planet.


Related media


Video releases


Because of hardships during filming, this episode ended up being heavily re-written from its original concept to accommodate changes in the production schedule. Several perceptible continuity issues ended up in the final cut due to this.


Episode images

Adaptation images



published order
Previous episode:
TOS episode produced Next episode:
Tomorrow is Yesterday
Previous episode:
Errand of Mercy
TOS episode aired Next episode:
The City on the Edge of Forever
Previous story:
first story
Star Trek 10
Next story:
The Empath
chronological order
Previous Adventure:
A Vulcan, A Klingon, and an Angel (backstory)
Pocket Next Adventure:
Tomorrow is Yesterday
Previous Adventure:
Voyages of the
USS Enterprise (NCC-1701)
(2264 to 2270)
Next Adventure:
Tomorrow is Yesterday

External links

  1. This character was not named in the episode but the same actor, wearing an officer's Starfleet uniform, was addressed as Brent in TOS episode: "The Naked Time". The same actor also played the character of Vinci.