American reporter Larry Stanford (Peter Dynely) is sent to Japan to do a story on the experiments of a reclusive scientist. Once at the scientist's mountain laboratory, the scientist decides to use Stanford for an experiment and injects him with a mystery serum, which causes the American to transform into a two-headed monster and go on a violent rampage. A Japanese-U.S. co-production filmed entirely in English, THE MANSTER is a creepy and influential horror oddity.