The planet Ariel gives the Moon an atmosphere, which is fun for a while until strong winds bring down an Eagle crew.
OH THE NOSTALGIA! In French, this was called "Le dernier crépuscule" ("The Last Twilight").
REVIEW: It's actually a lovely idea. The Moon could very well get trapped in a star's orbit - this is something they would face any time they crossed a star system - and be "stuck" with that new astronomical address. In this case, there seems to be habitable planet there, Ariel (why have all the planet names on this series been poached from actual moons in our own system?), but its natives don't want dirty humans to stink up the place so they send Alpha terraforming technology, whether we want it or not. Suddenly, there are blue skies and sunshine on the Moon, normal gravity (that particular bit of science isn't rock solid), rain and even the promise of things growing. It's great to see the Alphans let their luscious 70s hair down, get some sun, play badminton, and fall in love, reusing lunar sets in a new light, and always, the wind blowing.
But the people of Ariel have gone a bit too far. If they'd just given the Alphans a fertile Moon, that would have been fine, but the episode is actually quite smart - give or take openable windows on the base - in showing what would happen if the Moon gained at atmosphere overnight. There is so much dust on the Moon, air currents would spread it everywhere, burying things and eroding machine parts (the Eagles fail one after the other). If it rained, every crater would become a lake, including the one Alpha was built in. If the Moon failed to achieve stellar orbit, that atmosphere would freeze and come crush the base. The people of Ariel didn't think things through. What they sent was never going to satisfy the Alphans; they'd always need to check out the planet. Maybe that's why the spiked the manna, strange foodstuffs growing on the Moon, highly nutritious, but also hallucinogenic. When Paul is dosed, he starts to rant and rave about building a new civilization and staying put, just like the people of Ariel would want them all to.
Obviously, the elation they all feel when the Moon is terraformed must give way to sadness when their world-ship fails to achieve orbit, but the title (and format, let's not kid ourselves) kinda told you that would happen. At least they found a way to use the manna going forward, removing the groovy acid from it. Paul thinks it was still a "trip" though. (Oh Paul, you musician, you.) Hopefully, his relationship with Sandra will continue. She can forgive violent assaults caused by spiked manna, right? They're still together, watching the alien sun set, after all. It's not as overt, but Koenig's drive to rescue Helena (and the others, sure) seems to come from a romantic place as well. She can take care of herself, of course. It's great that though she's starving, she still mistrusts Paul's eagerness that she eat the new food, but not all her solutions are brainy. Sometimes, you've just got to blow the crap out of an Eagle with a laser bazooka to get some attention, you know?
REWATCHABILITY: Medium-High - I like Space 1999 best when it focuses on its characters' reactions to living in space, rather than the plot of the week. This one is actually well thought-out on both fronts.