In a clever, gender-bending twist on the classic Robert Louis Stevenson tale, the research done by Dr. Henry Jekyll (Ralph Bates) in the field of artificially-induced human longevity involves experimentation with female hormones. When he partakes of his own formula and the inevitable Jekyll-into-Hyde transformation takes place, he changes into a ravishing female version of himself (famed "B"-movie siren Martine Beswick). Claiming to be Jekyll's sister, Ms. Hyde is lovely but lethal: she uses her alluring charms to seduce men then kills them and absconds with their bodies for use in further experiments.