Stardate: 3087.6 - 3088.7

Original airdate: 3/30/1967

Writer: Don Ingalls

Director: Gerd Oswald

Guest stars:

Robert Brown            Lazarus, mysterious human who plays around with the crew and his alternate self. One is logical, calm, mild, rational: the other, urgent, anxious, murderous, vengeful.

Janet MacLachlen        Lt. Charlene Masters, engineering
Richard Derr            Barston (?)
Eddie Paskey            Lt. Leslie, helmsman

Crew Appearences: Kirk, Spock, McCoy, Uhura
No Sulu, Checkov or Scotty
Security Team: 4 redshirts

Story

"The Alternative Factor" was the 20th episode shot but was to aired as the 26th. The Enterprise encounters two versions of a man named Lazarus. He reveals that his ship is a time machine and he is a time traveler, chasing a 'monster' that is really his anit-matter universe self. Lazerus's keep going through a rip or door between the two dimensions, and if they meet means certain annihilation of both universes.Its the anti-matter Lazerus who martyrs himself to keep the 2 universes safe.

Takes place on an unnamed planet in a system that was being surveyed and charted when the episode occurs.

Star Base 200: next destination of the Enterprize when Star Fleet announces a Code Facter 1, invasion status.

Warp in space: when 2 universes (matter and antimatter) meet at a corridor for entry and exit. A magnetic valve that keeps eternity from blowing up.