**SPOILERS** The biggest problem with THE BEAST WITHIN is that about a third of the story appears to be missing. There seems to be an entire subplot about the man/thing who rapes Bibi Besch in the opening of the movie that is just not there. I have heard rumors over the years that an entire reel of the negative was lost or destroyed before post-production began, and they did not have the money to re-shoot the lost footage. If this is true, it explains a lot.
One night in the late fifties, a young newlywed couple (Besch, Ronny Cox) get stuck on the side of the road. Cox runs for help, while Besch senses that someones lurking in the woods, so, naturally, she goes to investigate. A monstrous humanoid creature appears and rapes her.
Seventeen years later...
Cox and Besch have a seventeen year old son (Paul Clemens) who is suffering from some mysterious illness. Knowing that there's a real chance the father is Besch's attacker from years earlier, they go to the small Southern town they were passing through to investigate, hoping the key to curing Clemens may lie there.
They venture to the town, but Clemens follows them, and starts picking off men who knew the man/thing that raped Besch. As best I can figure, the guy ("Billy") was so rotten and evil that he eventually turned into a monster, sort of a giant cicada. His buddies locked him up, but he escaped and raped Besch. Now seventeen years later (get it?...seventeen years?...cicadas?...no neither do I), he has come back in the form of Besch and Cox's son, apparently to seek revenge for being locked up in that musty old basement.
This may have made sense at some point, but it sure doesn't anymore. There's a lot of Southern Gothic mumbo jumbo, and people repeatedly mention cicadas. The one thing they don't talk about is exactly WHY "Billy" turned into a cicada in the first place, or exactly why he's p***ed off at all these people. I would almost be willing to accept this movie on an existential level, but there's too much exposition pointing at something more going on for that approach to work.
However, if you can get past the incoherent plot (or lack thereof), THE BEAST WITHIN is actually a pretty weird little flick. The acting is amusingly overwrought, there's some great, over-the-top music by Les Baxter, and some impressively gruesome gore sequences.
The infamous highlight of the movie, though, is the grotesque transformation scene. Clemens lays strapped to a hospital bed, while a dumbfounded group of people stand there like idiots and watch him slowly turn into a monster. His head swells up like a balloon, his tongue grows to an enormous size, his eyeballs grow into huge orbs. It's pretty freaky and genuinely frightening.
He then breaks out of the hospital and rapes a local girl, thus starting the whole vicious cycle all over again. The movie ends on a decidedly grim note, with Mom blowing his head off with a shotgun as he tries to kill Dad. The fact that he's not really her son anymore doesn't seem to console her as she breaks down. The sheriff comes walking up carrying the girl, now impregnated by "Billy". The end credits unceremoniously roll.
It's all pretty nonsensical, but the movie is bizarre enough to hold my attention. If anything, it's a nice reminder of old school horror. It may not be very good, but they just don't make them like this anymore.