Betty Boswell, with the help of her grandfather, Stephen, invents a helmet that allows the wearer to see other people's thoughts. Worried that it could be dangerous to test out on herself, Betty decides that they should find someone gullible enough to try it out on first. The following day at school, she tricks her best friend, Nancy, into coming over for just that purpose.
The device is a success, and things are going great, until Nancy passes out after accidentally reading the thoughts of Betty's grandfather (who happens to be fantasizing that he's raping the young girl at that moment). Stephen starts to panic, in contrast to Betty, who suddenly gets an exciting idea. She proposes to her grandfather that he rape Nancy while she's still unconscious, stating that she feels bad that he's been without a companion for so long. Shocked, Stephen immediately dismisses her words, adding that even if he were to attempt it, the girl could wake up at any moment. Betty sees his point, and promptly uses a chloroformed rag to ensure that Nancy stays knocked out cold. When Stephen further protests that the girl would know her virginity had been taken, Betty again counters that as a classmate had taken her virginity earlier in the year, adding "I know it to be true. I'm her best friend, after all!"The granddaughter finally convinces Stephen to go through with it, and steps outside the room to give him some privacy. However, the real reason that she orchestrated such a plan was so that she can read her grandpa's mind and to see what it's like to rape someone herself. Betty does just that, and concentrates so indulgently that she fails to notice two side-effects that are gradually being produced by the helmet. Firstly, her brain begins to increase in size - so much so that it destroys her skull and grows outward from it. Secondly, her newly-developed brain causes the air molecules near her groin to solidify into the shape of an invisible phallus; one that allows her to feel every sensation, as if it were a real male member.
Just as Stephen is finishing up with the girl, Betty bursts back into the room, overcome with lust. She then throws herself onto Nancy and rapes her classmate to death. Basking in the aftermath, she is not only devoid of guilt for what she has done, but vows to her grandpa that she wants to do it again and again, to as many girls as possible. She adds that she also plans to keep increasing her intelligence far past the already remarkable level it is at.Frightened and horrified by how out-of-control the situation has now become, Stephen tries to speak some sense into his granddaughter. Betty will hear no such talk, however, and murders him on the spot. Realizing that she can't go out into the world looking as she now does, the girl breaks into her next door neighbor's house and steals one of her wigs. When the neighbor returns to her home, Betty kills her as well, and then proceeds to make her way across the country; eager to put her new goals into action.
In London, Betty continues to abduct, rape, kill, and then extract brain matter from random young women, all while reading every book on biology that the library has to offer. Two years pass, and Betty's experiments are finally met with success; she has not only increased her intelligence, but has gained the power to control matter telekinetically. She decides to test out this new-found ability by mentally creating a sadistic massacre out on the city streets, using random pedestrians. Realizing that she can now do whatever she wants, Betty deeply immerses herself in fantastic daydreams for the future. In doing so, however, she fails to pay attention as she walks onto a busy street, and is both struck and killed by a vehicle.