The story begins with a chilling display of sociopathy when Jill, upon learning that her husband Terry has started cancer treatment, immediately visits a dating agency. She is seemingly content and relieved by the prospect of his impending death. In a calculated move to gain social capital, Jill begins presenting herself as a widow to her co-workers and neighbors, despite the fact that Terry is still very much alive. This allows her to manipulate those around her and garner an immense amount of unearned sympathy.
Jill sets her sights on Don, a family doctor whose wife, Cathy, suffers from multiple sclerosis and frequently relies on a wheelchair. Under the guise of providing compassionate care for Cathy, Jill systematically integrates herself into their household. While playing the role of a supportive friend to Cathy, she simultaneously flirts with their son, David, and attempts to undermine the marriage to seduce Don.
The plot thickens when Jill discovers that Terry is actually recovering from his illness. Unable to let her "widow" persona slip, she admits him to a hospice and informs her entire social circle that he has passed away. She goes so far as to stage a twisted and theatrical funeral, ensuring that all eyes and sympathy are focused solely on her. When Terry eventually manages to leave the hospice and return home, Jill does not welcome him; instead, she imprisons him in a spare room. She subjects him to starvation and brutal physical abuse, all while claiming that these horrific acts are actually intended to aid his recovery.
As Don and Cathy plan a move to Hopperton—a Christian retreat known for its high population of lesbians—Jill decides to leave a lasting impression. She organizes a farewell coffee morning for them, which she abruptly transforms into a spectacle by performing a pole dance routine, leaving the neighboring guests watching in absolute horror. Shortly after the party, realizing that Terry is a loose end that must be tied off, Jill goes upstairs and smothers him with a cushion.
Three weeks later, Jill has successfully maneuvered her way into the mansion of Glen, a man who is wealthy but notably dimwitted. Her security is threatened when Glen invites Gordon, the local vicar and a friend of Jill, to arrange a wedding. Fearing that her crimes are about to be uncovered, Jill confesses the murder of her husband to Glen. In a ruthless move to silence the vicar, she laces dishes of Angel Delight with poison and encourages Gordon to eat. As Gordon chokes on the toxin, Jill manipulates Glen, telling him that if he truly loves her, he must take the blame for the deaths of both Gordon and Terry. She persuades him to make a full telephone confession to the police. To finalize the plan, she suggests a joint suicide pact via the poisoned dessert. However, when it is her turn to eat, she coldly declares:
> "I'm not really hungry"
Glen collapses as the poison takes effect, and Jill immediately contacts Don. While Glen survives the attempt on his life, his previous confession leads to his incarceration in a secure unit for the criminally insane.
Driven by the need to inherit Glen's fortune to fund her pursuit of Don, Jill agrees to marry the incarcerated man while simultaneously plotting his death. She steals a caravan from a woman named Linda and tracks Don and Cathy to Bude, Cornwall. The couple is attending a New-Age retreat called The Trees to repair their marriage. During their journey, Don and Cathy accidentally run over a therapist named Floella Umbagabe. They hide her body in their caravan, providing the perfect opportunity for Jill to assume Floella's identity to infiltrate the retreat.
Upon discovering that Cathy is pregnant and that Don intends to have a vasectomy, Jill realizes her window of opportunity to secure Don permanently is closing. She attempts to illegally obtain a semen sample from Don before his surgery. When this fails, she turns her attention to Cathy and Don's 12-year-old son, Bruce. After Bruce rejects her advances, Jill maliciously claims to his parents that the boy raped her and that she is pregnant with his child.
Meanwhile, Glen has managed to tunnel out of his cell and tracks Jill to Cornwall. Simultaneously, Floella Umbagabe—who has survived the accident—arrives at The Trees and exposes Jill as a complete fraud.
Eleven months later, Cathy has given birth to a baby girl named Abigail. Don expresses a desire to move to Spain with a woman named Sue to start a new life. Overhearing this, Jill mistakenly believes Don is talking about her. At this moment, Glen finds Jill and threatens her life, but she once again deceives him by claiming she is pregnant with his child, bringing him back under her control.
The climax occurs when Jill's web of lies is finally exposed. She is chased to the edge of a cliff, where Cathy confronts her regarding the fake pregnancy and the repeated attempts to seduce Don. A violent struggle ensues while Don and Sue are intimate on the rocks below. In the chaos, Cathy's wheelchair is hurled off the cliff, accidentally killing Sue. Cathy then pushes Jill off the cliff; however, Jill's fall is broken by a trampoline and then by Don himself. The story concludes with Cathy being arrested and taken away by police, while Jill makes her escape in a speed boat accompanied by both Don and Glen.
