Joan Collins plays a former dancer named Lucy who worked at a seedy nightclub where she was in an act with a frolicking male dwarf named Hercules (George Claydon). Lucy used to flirt with Hercules harmlessly, she thought, but he took her seriously and made a pass at her one night, which she sternly rejected. Humiliated and angry, Hercules then leaps out in front of her and declaring, You will have a baby, a monster, an evil monster conceived in your womb, as big as I am small, and possessed by the devil himself!
Lucy thought nothing of it at the time, but after months pass and Lucy leaves the dancer business behind, she gets married and does give birth to a child. As soon as he is born, the little tyke is raking his fingernails down Lucy's face, and before long he's wrecking his playroom, ripping the heads off his dolls, spitting at people, and dropping dead mice into teacups. Things get worse as people around the baby are murdered.
Lucy's sister-in-law, a Catholic Nun suspects that the baby is possessed by a demon. Turns out, she's right.
The Devil Within Her provides examples of:
- Bow Chicka Wow Wow: 70's porn music plays during the lengthy sex scene between Lucy and Gino.
- Demonic Possession: The baby Nicholas.
- Depraved Dwarf: Hercules is first introduced trying to grope Lucy while in her dressing room while they are both working at a burlesque house.
- Gypsy Curse: While a dwarf, not a gypsy, Hercules' curse to Lucy is essentially this and treated as such.
- Hollywood Exorcism: Lucy's sister-in-law performs one at the film's climax. It has the side-effect of killing Hercules, presumably through sympathetic magic - as the demon goes, so he goes.
- Kill 'Em All: Except for Lucy's sister-in-law, and her friend played by Caroline Munro, the baby eventually ends up killing every other character (including his own mother).
- The Mockbuster: Often compared to Rosemary's Baby (One of the original titles was Sharon's Baby), The Exorcist, and The Omen (1976). all of which had very similar plots, although in the latter case, The Devil Within Her predates it by a year.
- Nuns Are Spooky: Lucy's sister-in-law is a nun, and she's in a horror movie, but other than she's one of the good characters.
- Title Drop: At one point during the birth of Nicholas, the doctor says "It doesn't want to be born," which was one of the film's titles.
- Troubling Unchildlike Behavior: Everything baby Nicholas does.