The expression "jumped the shark" refers to when a TV series, out of desperation, does something so atypical and so wrong to try (in vain) to rejuvenate the series. To fans, it marks the beginning of the end. The exact reference is when on "Happy Days", Fonzie jumped a tank full of sharks with his motorcycle--a very dumb and desperate gimmick to say the least.
As far as "Space: 1999" goes, it was obvious during season one that the series had problems. Despite a large budget and international syndication, the show had trouble attracting an audience. Some of this was due to the show's strange plots but much of it was the sterile and static nature of the show. In a completely misguided attempt to rejuvenate the show, the series decided that instead of making a few minor tweaks, it changed a lot--too much. It essentially jumped the shark by losing some of its stars (the big loss was Barry Morse), changing its music, adding a new character that could shape-shift into animals(!!!), bringing in "Star Trek" producer Fred Freiberger and 'jazzing up' the scripts. What this jazzing up entailed was actually much cheaper special effects (season one had been very good--two was just god-awful) and lots of silly monsters in latex suits! In addition, the show (with Freiberger) had one more ace up their sleeves--ripping off scripts from the original "Star Trek" series!! Sadly, they didn't even bother ripping off the good shows--mostly just the terrible ones.
In this episode, the script clearly is a rip-off of one of the very, very worst "Trek" shows, "Who Mourns for Adonis?". While there were of course a few small differences, it is practically impossible to see the two shows and not see the similarities--especially at the way it ends. Because the show is one giant rip-off, I can't give it a score any higher than 3.
The show's 'big four' (Commander Koenig, Dr. Russell, Mya and Tony) are brought to a new world by a guy claiming he's God. By the way, beware--this sort of plot totally sucks ("Star Trek V" is a great case in point) and you KNOW that ultimately the guy can't be God! In this case, a very, very technically advanced and god-like guy brings them to the planet to have them create a whole new population--sort of like having two Adams and two Eves! Despite his powers, Koenig and the rest are not about to worship this guy nor stay on the planet if they can help it. Been there, done that.