As early as he began pitching the idea in March 1964, the original Star Trek series was merely an idea in the head of television producer Gene Roddenberry. He took his idea of a "Wagon Train to the stars" - a five-year voyage through space in search of new life - to MGM Television (producers of his previous show, The Lieutenant), who rejected it. He also pitched the idea to CBS, who also rejected because they already had Lost in Space in development. However, NBC picked up the idea, and eventually ordered two pilots.
"The Cage"
The first pilot, "The Cage", was filmed between November 1964 and January 1965. Here, Jeffrey Hunter plays Christopher Pike, captain of the U.S.S. Enterprise, while Mr. Spock was played by Leonard Nimoy and Majel Barrett (Roddenberry's future wife) played Number One. In this pilot, the Enterprise crew travels to Talos IV in search of the survivors of the crash of a previous starship. However, this turns out to be an illusion, and Pike is captured by Talosians, who stick him in a cage and put in him various imaginary situations. While this happens, the Enterprise crew attempts to break him free, which they eventually do.
When this first pilot was screened to NBC executives in February 1965, they rejected it for being "too cerebral". However, they liked the concept enough to make the unusual decision of ordering a second pilot, but only Nimoy remained for this next pilot. After being rejected by Lloyd Bridges and Jack Lord, Roddenberry eventually found his new captain in struggling Canadian actor William Shatner.
"Where No Man Has Gone Before" (original cut)
The second pilot follows the U.S.S. Enterprise in an attempt to find the remains of the long-destroyed Valiant, crosses a strange barrier at the end of the Galaxy. This results in Lt. Cmdr. Gary Mitchell (Gary Lockwood) - and later, Dr. Elizabeth Dehner (Sally Kellerman) - developing godlike powers after being hit by the storm. The rest of the crew is concerned over what Gary is becoming, and so Captain James T. Kirk (Shatner) decides to maroon him on Delta Vega. Unlike the series itself, this pilot ran 55 minutes and featured Quinn Martin-style act bumpers, but when the episode aired as part of the regular series, it was recut.
NBC's reaction to the new pilot was improved, and Star Trek was finally given a series order. It debuted September 8, 1966, with the episode "The Man Trap", airing Thursdays at 9:00 PM Eastern time after Daniel Boone. The rest, of course, is history.
Release
Roddenberry had a black-and-white workprint of "The Cage" that he'd show at sci-fi conventions, and footage was eventually used in the two-part episode "The Menagerie". However, the latter eventually forced the original camera negative for "The Cage" to be disassembled, making it partly lost. The footage that made it into "The Menagerie" was combined with scenes from Roddenberry's workprint and released on VHS in 1986 by Paramount Home Video. A full-color presentation was not shown until 1988, during the TV special The Star Trek Saga: From One Generation to the Next. "The Cage" has since been released on DVD and Blu-Ray, in its full-color form.
As for the alternate cut of "Where No Man Has Gone Before", it has been circulated at conventions via bootleg videotapes. In 2009, a print was eventually discovered by a German filmmaker, who sent it to CBS/Paramount. The pilot was then remastered, and the alternate cut was included as a bonus feature on the Season 3 Blu-Ray in 2009.