Made by the then 23-year-old director Michael Reeves, who died after making only one more movie, the notable Witchfinder General (1968), this is an unusual horror film concerning an elderly couple who conduct experiments with mind control, hoping to experience the lost excitements of youth through their subject, a young man whom they have persuaded to become a guinea-pig in the name of science. Once the process has begun however, a conflict ensues between the couple, the woman urging their subject to commit crimes in the pursuit of even greater thrills against the wishes of her husband, which in turn results in a horrific comeuppance for both, the price exacted for meddling in things beyond the province of humanity. With the casting of veteran actor, Boris Karloff in this swinging sixties setting, this is a rare example of the merging of two styles of horror movie-making, the old school which Karloff represented almost gone by the late sixties, a new, grittier contemporary genre waiting to succeed it.