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. A much more interesting twist comes when Jekyll finds himself falling in love with the girl next door (Susan Brodrick), while simultaneously lusting after the girl's brother (Lewis Fiander) as Hyde. Although Brian Clemens' script manages to exploit this unique premise for shock value, the story fumbles where it counts, failing to fully explore the implicit questions of sexual identity which haunt Jekyll's psyche and burst to the surface when Hyde is on the prowl.