The first thing to point out is that this is set nowhere near the year 2889, and neither is it to be confused with the Jules Verne story of the same name. This is one of a series of made-for-TV movies churned out by Buchanan's Azalea Films during the sixties, mostly remakes of AIP pictures from a decade previously - the most famous of these is Mars Needs Women. In this case, the target is 1955's The Day the World Ended, which wasn't a great movie to begin with, and is notable now solely for being one of Roger Corman's first genre entries. The scenario see retired sailor John Ramsey (Fletcher) holed-up with his daughter Joanne (Doherty) in their remote compound, awaiting the arrival of Joanne's fiance. However, it appears someone posted the location of their compound on Facebook, or whatever the 60's equivalent was, as a steady stream of visitors show up, riding roughshod over Ramsey's protestations that he has food only for three people. By the time the check-in desk closes, they've gone from two to seven, even if one of the new arrivals has taken a beyond-lethal dose of radiation and is confined to bed. He makes a miraculous recovery, though is unwilling to take a shower and has a disturbing craving for raw meat.
Meanwhile, another mutant is roaming the (remarkably well-kept) grounds, including a swimming pool in which Joanne likes to take a dip, drawing the leering attentions of small-time punk Mickey (Feagin), who is looking for an alternative to his stripper moll. It's up to heroic geologist Steve (Petersen) to fend him off, and generally act like the mysteriously-absent fiance. The radiation has apparently gone through a million years of evolution in weeks, and the monster is now invulnerable - there's hints of an entire clan of such beasts beyond the cliffs, but presumably for budgetary reasons, we only ever see one, and it's not feeling well. However, there is a flaw in its armor: without wishing to give the game away too much, one wonders whether M. Night Shyamalan stole it for Signs, though at least here, the creature is not a technologically superior, trans-stellar species which should really know a good deal better.
That may be about the only area where the film scores points, as in every other way, it's a bargain-basement production. A concrete swimming-pool makes a very poor stand-in for a natural spring, and to describe the mutant as "laughably cheap" would be an insult to laughably cheap things - at one point, when reaching into a trap for a rabbit, you can clearly see its fingers bending back against the cage, like the rubber they are. Probably wisely, it's relegated to a supporting threat role, with Mickey's sleazy shenigans a bigger danger to what remains of humanity - though feminists may feel that way about Ramsey demanding the women should start having babies as soon as possible. Much as with his statements about there only being enough food for three, everyone ignores him. Hey, no harm in trying. It would likely have made for a more interesting - albeit not perhaps TV-friendly - movie if they'd listened.