Johnnie Tallent, Paul Nicholas, is a callous young man who lives with his elderly, invalid grandmother, Gran Alice Tallent, [Mona Washbourne]. He is lazy and unmotivated, spending most of his time taking care of her merely to stay in her good graces so that he can inherit her small fortune and valuable house after she dies. His life outside of Alice centers on his girlfriend Jill Standish, [Vanessa Howard], an even more callous travel agent who shares his mercenary outlook.
Jill encourages Johnnie to take active steps to speed up Alice’s death so that the two can marry and retire on the fortune. They hatch a plan to induce a heart attack in Alice by gaslighting her, effectively murdering her while leaving no evidence of the crime. To make their manipulation feel credible, Johnnie slowly convinces Alice that London’s twenty-somethings, convinced that the elderly have become a drain on society, are planning a youth-led revolution with the aim of either killing the elderly or placing them in internment camps. Johnnie also gains access to Alice’s newspapers and television viewing, using selected stories and footage of protests to fuel her fear and paranoia.
To seal the illusion, they cover the wall outside Alice’s bedroom with ageist graffiti, a detail that reinforces her sense of danger. Over several weeks, Alice becomes increasingly paranoid and reclusive, and her health worsens. Jill leverages her position at the travel agency to arrange a large parade that will pass by Alice’s house one afternoon; that morning, Johnnie tells Alice that the revolution has begun and that rioters are going door-to-door looking for elderly people to kill or intern. When the parade arrives, Alice, already in a panic, suffers a heart attack. Johnnie allows her to die before calling an ambulance.
At Alice’s probate attorney’s office, the couple learns that she has left a codicil in her will: as long as he remains in a relationship with Jill, Johnnie is only entitled to the house; to inherit any of her money, he must sever ties with Jill and marry another woman. Johnnie and Jill initially try to find well-paying jobs to keep the house, but neither is willing to work, and soon their electricity, gas, and water are shut off. They hatch a grim plan to date and marry an impressionable young woman in quick succession, so Johnnie can collect the inheritance and then end the relationship to be with Jill.
This plan is destabilized when Jill grows violently jealous as Johnnie appears to develop genuine feelings for one of their targets. The couple’s tension erupts into a desperate struggle, culminating in Jill being stabbed in the abdomen. She staggers into the street as neighbors call the police, and the sobbing Johnnie crawls toward Alice’s room, crying for his grandmother as the authorities arrive.
