I love Rosemary's Baby (the book more than the film), though I disliked the sequels. The idea of a prequel about Terry Gionoffrio sounded fascinating. My biggest problem was that everything we know about Terry from Rosemary's Baby (save that she dies in the end) was contradicted in this script. As one of the simplest examples, when the girl who undoubtedly is supposed to be Rosemary enters the laundry room, Terry is distressed and frightened. Had we seen them converse (which we do not), Terry would not have been saying they were 'the most wonderful people in the world, bar none.'
For us to see the characters we know well from the books could have been fascinating. It's perfectly credible that someone other than Terry or Rosemary could have been chosen to bear Satan's son in the past. Yet it seemed very contrived. At certain points, I wondered 'is this a spoof? I am not sure if it is." (I know there weren't scans at the time, but, had Terry carried even a foetus with horns, claws, and a tail, the autopsy would have been questionable - and Dr Sapirstein certainly would not have suggested, earlier, that Terry be hospitalised.)
I watched it with a Paramount+ free trial - but I'm very glad I didn't pay a rental fee. It is not as terrible as "Son of Rosemary," or the television film which was worse still - or even as bad as the mini-series which distorted the themes and plot. Yet it seemed it was a film that was poor - which could have been good.