Tom Burman's innovative prosthetic special effects (used to greater effect in later horror films) are the saving grace of The Beast Within. The premise concerns a couple honeymooning in Mississippi. Caroline (Bibi Besch) is brutally raped by a hairy, sub-human monster and gives birth to a child named Michael (Paul Clemens), who appears normal until he hits his teens. At 17, Michael begins to feel strange, and his parents take him back to Mississippi to find out if his problems are related to Caroline's long-ago rape. Once in Mississippi, Michael transforms into a ravenous insect-like creature that roams the countryside, disemboweling innocent victims and feasting on their torsos.
~ Paul Brenner, All Movie Guide