In the late 1800's, Boris Karloff has discovered a way to use nuclear power. He creates a beam weapon and blows up a big rock. Outer space aliens are scared and one alien who looks a lot like James Cameron with a big fake nose only this one isn't fake lands and brings other aliens who take over the bodies of Karloff and his assistant. The assistant is a Jack the Ripper style killer who has done a few nasty murders which have riled the townsfolk. The body-snatched bodies become radioactive and start killing flowers and other stuff (but don't die themselves) and some stuff happens and in the end Karloff destroys the beam weapon equipment (and his lab and house), then the aliens leave and warn us that if anyone ever does it again there will be trouble.
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