In 1978, the self-absorbed and fading actress Madeline Ashton performs in a Broadway musical, inviting her long-time rival, the timid aspiring writer Helen Sharp, along with her fiancé, the renowned plastic surgeon Ernest Menville. Captivated by Madeline's allure, Ernest breaks his engagement with Helen to marry her instead. Fast forward seven years, and a lonely, overweight, and despondent Helen finds herself in a psychiatric hospital, consumed by thoughts of exacting revenge on Madeline.
Jump ahead another seven years, and Madeline and Ernest lead a lavish yet miserable life in Beverly Hills. Madeline is plagued by the reality of aging and waning beauty, while Ernest, now an alcoholic, has devolved into working as a reconstructive mortician. An invitation to a party celebrating Helen's new book sends Madeline into a frenzy for beauty treatments. In her desperation to reclaim her youthful appearance, she receives a card from Lisle Von Rhuman, a mysterious and wealthy socialite known for her rejuvenation services.
At Helen's party, Madeline is taken aback by Helen’s transformation into a slim, glamorous woman despite being fifty years old. Overwhelmed by jealousy, Madeline listens as Helen places blame for Ernest’s career demise squarely on her shoulders. After a disappointing encounter with her younger lover, Madeline drives to Lisle's opulent mansion, where she learns that Lisle, appearing youthful at seventy-one, offers her a potion that promises eternal beauty and life. Madeline consumes the potion, regaining her youthful glow, but Lisle cautions her to remain out of the public eye after ten years to avoid drawing suspicion about her apparent immortality.
As tensions mount, Helen entices Ernest into a scheme to murder Madeline. Upon returning home, Madeline, dismissive of Ernest, finds herself pushed down the stairs, leaving her with a broken neck—yet she miraculously survives. Ernest, believing this to be a miracle, repairs her body at home. However, when Helen learns about their conspiracy, the plot twists as Madeline, in a shocking turn of events, shoots Helen with a shotgun. Although gravely wounded, Helen reveals she too has consumed Lisle's potion. The two women engage in a chaotic confrontation but ultimately come to an understanding and reconcile. Feeling dejected, Ernest attempts to leave, but both women insist he tends to their bodies first. Realizing the need for consistent care, they manipulate Ernest into drinking the potion to secure his assistance indefinitely.
The women knock out Ernest and take him to Lisle, who entices him with the potion in return for his surgical expertise. Despite the allure, Ernest turns it down, fearing he'd outlive anyone he loves and endure the physical repercussions he has witnessed Madeline and Helen suffer. He escapes with the potion but finds himself cornered on a rooftop. As the two women urge him to drink the potion for survival, he comes to the painful realization of their selfish intentions and discards the vial. Ernest survives the fall into Lisle's pool, leaving Madeline and Helen trapped in their reliance on one another for companionship and maintenance, bound to eternity together.
Thirty-seven years later, Madeline and Helen attend Ernest's funeral, where he is honored for achieving true immortality through a rich and fulfilling life, surrounded by children and grandchildren. Now grotesquely altered shadows of their former selves, with cracked, peeling skin and putty covering their decaying features, the pair mock the eulogy. When Helen stumbles and pulls Madeline down a flight of stairs, their bodies disintegrate, leading Helen to humorously ask if Madeline remembers where they parked their car.
