Dr. George Dumurrier (Jean Sorel) runs a medical clinic with his brother, Henry (Alberto de Mendoza), that engages in ambitious research but is failing financially. George's wife, Susan (Marisa Mell) has severe asthma which forces her to be home most of the time, usually alone. George and Susan do not have a happy marriage. George tells Susan that he has to go away for a few days to take care of the clinic's financial matters to Susan's dismay but he promises to hire a medical attendant for Susan to give her around-the-clock care. The young nurse (Malisa Longo) gets a tutorial from George on Susan's medicine. In identical bottles, one medicine is necessary for her asthma attacks and in the other, a sedative is given to Susan at bedtime to help her sleep. However, if Susan were to receive the sedative during an asthma attack, then the results could be disastrous. With the nurse instructed, George leaves on his trip to meet his mistress, Jane (Elsa Martinelli), and the two have their final fling: Jane doesn't want to continue the affair, because Susan will not agree to a divorce. She doesn't want to share her lover. Jane makes a dramatic exit by train, but George beats her train to the destination. He's waiting for her, and the two embrace. While George and Jane are enjoying their evening together, Henry calls George to tell him that Susan has died (from the nurse's negligence). The nurse is nowhere to be found. A few days later, George learns that he's the beneficiary of Susan's million-dollar life insurance policy. Finally, one evening, George leaves Jane at dinner, after receiving an anonymous phone call, to go to a cabaret whose headliner is a dead ringer for Susan, Ms. Monica Weston (Marisa Mell).
The initial exposition and set-up for the mystery in Lucio Fulci's Perversion Story (Una sull'altra) (1969) is very well executed and intriguing, and with this exposition and set-up Fulci can take his viewer in three directions with his narrative (story and screenplay which he helped script): Sorel's George has a strong motive for murder (unhappy marriage, another lover, beneficiary of a large policy, and failing financially with his clinic), so Fulci could play out his film as a deductive mystery to uncover whether George is a murderer; the viewer could also follow George and Martinelli's Jane after their first encounter with Monica and learn the backstory and identity of Susan's doppelganger; or finally, the viewer can plumb the depths with George and Jane, as the film's English-language title suggests, as each indulges a morbid curiosity with a growing fascination with the highly-sensuous and mysterious Monica. Fulci ultimately settles upon the former for his narrative, unfortunately, but he also shows in scenes flashes of what could be, if he chose to indulge his viewer in the latter two narrative storylines.
Dr. George Dumurrier is a cold and aloof character, and Sorel's performance is appropriately stiff. Fulci wisely plays to both, and expectedly, when George first encounters Monica whatever mystery he has hidden within him should start to reveal itself. George and Jane don't provoke Monica to get a reaction out of her during their first meeting nor does either reveal that she's a dead ringer for recently-deceased Susan. An insurance investigator following George can only submit to the police circumstantial evidence with their strongest piece being Monica's uncanny likeness to the dead woman. The police investigation is led by Inspector Wald (John Ireland), and instead of going after the suspect with the strongest motive, George, they focus their investigation on Monica by detaining her for questioning and searching her flat. There's no strong evidence linking either George or Monica to the crime. The young nurse who disappeared is a promising lead, but Fulci reserves a revelation for her character later on. As such, the police have really no clues, and Fulci doesn't give the viewer any real ones to go on either. At certain points, Fulci has to make leaps in logic, and the police have to conveniently dodge procedural problems in their investigation. As the mystery plays out, it becomes increasingly more incredulous and any fun trying to solve it disappears long before the film ends.
Marisa Mell's first appearance as Monica is striking: not only does she capture the viewer's attention with her powerful charisma and beauty but she also enamors George and Jane. The following day George pays a large sum of money to learn Monica's telephone number and arranges a rendezvous. George is still silent and aloof (which Monica comments on to a friend. He rarely speaks and when he does, he's crazy, she says). George and Monica have sex, and Martinelli's Jane later probes George on why he wanted to make love to Monica. Indeed. It's an interesting question, and the answer could possibly reveal some provocative and dark aspects of George's character (and themes of the film). Jane's investigation of Monica's identity borders, like George, on obsession (hinting, also, that something else is driving Jane). Martinelli and Mell have a fantastic scene together later in a photographer's flat: Fulci is able to create a tension between the two that alternates between both as adversaries to strongly-attracted lovers. Sorel's George and Martinelli's Jane give the film their best scenes when each is alone with Monica: so much potential and tension is created to only go flat.
Marisa Mell's performance as Monica makes Perversion Story worth seeing alone. Her previous performance in Mario Bava's Danger: Diabolik (1968) was a tremendous amount of fun, and she would go on later to make several notable films. Riz Ortolani contributes a fantastic jazzy score, and it's included with the Severin release on Perversion Story on DVD. While Lucio Fulci shows flashes within Perversion Story, he would be much more successful with his subsequent mysteries, such as Lizard In a Woman's Skin (1971), the excellent Don't Torture a Duckling (1972), and The Psychic (1977), for example.