Eunice, a complex and deeply troubled bisexual serial killer, embarks on a chilling journey along the desolate motorways of Lancashire, driven by an obsessive quest to find a mysterious woman named Judith. Throughout her journey, she persistently inquires at various filling stations, always asking the cashiers if their name is Judith. After a dead-end at one station, she steps outside and douses herself in petrol, on the brink of self-destruction. Just in time, the compassionate cashier, Miriam, rushes out and offers her refuge for the night at her apartment, where she lives with her disabled grandmother. On their way to the apartment, Eunice tantalizingly shows Miriam a bundle of letters she claims were penned by Judith.
Within the confines of the apartment, Eunice reveals an astonishing transformation as she strips off her clothes, displaying her body adorned with chains, piercings, and tattoos, embodying a vivid image of both pain and allure. As tension builds, she seduces Miriam, only to leave abruptly the following morning, marking her departure with a haunting message on the mirror: "you are not Judith," scrawled in toothpaste. Eunice then hitches a ride with a delivery driver named Tony, whom she seduces in the throes of passion before brutally murdering him and stealing his van.
Meanwhile, driven by her desire for adventure, Miriam abandons her grandmother to search for Eunice, catching a ride with her friend Mr. McDermott to track her down. When they reunite, Eunice reveals the horrific truth—Tony's lifeless body hidden in the van. To Miriam's surprise, she willingly assists Eunice in concealing the grim evidence, dragging the corpse out for burial. However, their bond is put to the test when Eunice leaves Miriam to deal with the aftermath alone at a restaurant.
As they travel together, the tension thickens. They decide to pick up two hitchhikers, Gary and his young daughter, Katie, who are en route to Morecambe. A detour to an amusement park leads to further complications as Miriam discovers another of Eunice's secrets—the body of a waitress named Angela hidden in the trunk. Faced with a moral crisis, Miriam refuses to let Gary and Katie continue their journey with them. Confirming her worst fears, Eunice manipulates the situation by tricking Gary into leaving to search for Katie, who had innocently strayed away. Haunted by the belief that she has now committed murder, Miriam is overwhelmed as Katie suddenly appears in the back seat, prompting her to frantically abandon the girl by the roadside.
On a desolate bridge, Eunice grapples with her own demons, revealing her bitter frustration at the world and God's perceived indifference towards her heinous acts. Seeking comfort, Miriam pledges to stand by Eunice, determined to help her transform into a better person. Yet, Eunice chillingly counters, asserting that redemption is an impossibility for her.
Their chaotic journey leads them to a truck stop, where they dispose of the stolen vehicle and strike up a conversation with Robert, a philosophical truck driver. In a twisted power play, Eunice invites Robert to engage in a sexual encounter with both her and Miriam. As Robert begins to seduce Miriam, Eunice, seething with jealousy, returns and violently bludgeons him to death with a stolen headlight before commandeering his truck. At the next stop, Eunice, in her usual unsettling fashion, innocently asks the cashier if her name is Judith. When the woman responds, caught off guard by Eunice's fervent display of affection, Eunice erupts into a rage, only to be pulled back by Miriam.
As they continue their harrowing journey, Miriam negotiates another ride with Mr. McDermott, who unwittingly becomes another pawn in Eunice's twisted games. In an act of misguided heroism, Miriam misinterprets a situation, resulting in a shocking act of violence against McDermott—leading Eunice to calmly declare that now they are both damned. Later, as they drive through the night, Miriam asks Eunice about her ultimate desire upon finding Judith. With a chilling calmness, Eunice reveals her intention to sacrifice Judith herself for a greater purpose.
As dawn breaks, Eunice perplexingly expresses her wish to become the sacrificial offering instead. Stripping off her chains, which symbolize her guilt and penance, she and Miriam find solace in one another, marching towards the ocean at sunrise. Amid the rising tides, Eunice recites biblical verses, surrendering herself as Miriam reluctantly prepares to drown her. The film concludes in a haunting image, as the sea envelops Eunice, leaving Miriam cradling her lifeless body in a poignant Pieta pose, filled with sorrow and despair against the backdrop of the unforgiving waves.
