The plot centers around the plight of the inhabitants of Moonbase Alpha following a strange accident that rips the moon out of Earth's orbit and sends it hurtling into interstellar space at superluminal speeds.
The series was the third attempt by the Andersons, best known for their "Supermarionation" series such as Fireball XL5, Captain Scarlet, Stingray, and Thunderbirds, to create a live-action series following the success of UFO. Although it retains a cult following, the reaction to the series among the both critics and sci-fi community is very divided, with some criticizing the acting, writing, and direction, and others regarding it - or at least the first season - as a classic of the genre. Both sides, though have consistently praised the series' visual effects.
As with most films and television series, many scenes were shot that did not make it into the final cut. With Space: 1999, however, many of these scenes appear to be missing, with some existing only in audio form and other seemingly being lost entirely. These include several alternate and extended scenes from the pilot episode "Breakaway", as well as a somewhat plot-relevant scene in "The Metamorph" revealing the fates of Professor Bergman, Morrow, and Kano. While the "Breakaway" scenes, which only exist in audio form, have been released online by YouTube user Spaceman042, the "Metamorph" scene has not resurfaced in any form. The latter is of great interest to the series' fandom as it explains the absence of these popular characters during the second season. It is unknown where the footage may have ended up, and whether they will ever fully resurface.