The film opens with Agatha Christie receiving a silver cup engraved for her husband, Archie Christie, who accepts the gift with utter disdain. The couple heads to a publicity event for Agatha’s new novel, The Murder of Roger Ackroyd, tailed by American reporter Wally Stanton. The next morning, Archie asks for a divorce, confessing that he loves his secretary, Nancy Neele.
That night, Agatha leaves the house and is involved in an accident. The following morning, her wrecked car is found, and the press swarms the scene, speculating about suicide after a note left for her secretary. Agatha travels to Harrogate by train and checks into the Harrogate Hydropathic Hotel (now the Old Swan Hotel), registering under the name Theresa Neele from Cape Town. In the lounge, Evelyn Crawley, another guest, notices Agatha’s torn stockings and muddy shoes, a detail Agatha later notes to herself as a potential tool to “use” Crawley.
The next day, Agatha begins treatments at the Royal Baths as the newspapers whip up headlines about her disappearance. The police question why Archie isn’t helping, discovering he’s gone to Harrogate with Nancy for her slimming treatments. Meanwhile, Nancy Neele — Agatha’s secretary — reveals that Agatha left her a coded message within newspaper advertisements.
The coded ad leads Stanton to the Harrogate hotel. He conspicuously checks in while Agatha is on the phone at the front desk, and in the billiard room he helps her make a winning triple bank shot. He introduces himself as Curtis Shacks Jr., a visitor seeking treatment for constipation. They spend the evening together, and he makes a pass at her, which she declines. In his room, he types notes about her behavior, quietly cataloging what he observes.
As Agatha researches spa treatments—galvanic baths and the Bergonic chair—she asks attendants how they avoid fatal accidents with the equipment. Stanton trails her to an electronics shop and begins to piece together what she is up to. She studies a Bergonic chair manual, experiments with the rheostat, and even impersonates a staff member on the phone to reschedule Nancy’s appointment earlier.
Agatha rigs the Bergonic chair while Nancy undresses for her treatment. When Nancy enters the room, Agatha calls out from hiding to say the nurse is not there, urging Nancy to turn on the electricity. With this deduction, Stanton races to locate the room; Nancy flips the switch, producing a massive spark. Stanton arrives just in time to see Nancy scream, and behind a curtain he finds Agatha seated in the Bergonic chair. The suicide attempt is thwarted, and Stanton resuscitates her with CPR.
The Christies claim Agatha suffered amnesia from the accident and remembers nothing. Later, Agatha visits Stanton as he packs, handing her a copy of the story he wrote and confessing that he cannot publish it. She helps him with his packing, and he admits his love for her. She explains that she will divorce [Archie Christie], hinting at a possible future with Stanton. As the couple depart at the railway station, the closing credits reveal that they divorced two years later and that Archie married [Nancy Neele].
Throughout, the narrative shifts between public scandal and intimate revelation, painting a portrait of memory, manipulation, and the fragile boundaries of trust. The tensions between fame, marriage, and desire drive a suspenseful arc that remains grounded in the characters’ choices and the consequences that follow.
