This crime/mystery/thriller was adapted by Jonathan Hales and Barry Sandler, .
Scenes were filmed at Twickenham Film Studios, Twickenham, London, England, and on location in Kent.
PlotSet in the fictional English village of St. Mary Mead, home of Miss Jane Marple (played by Lansbury), in 1953, a big Hollywood production company arrives to film a costume movie about Mary, Queen of Scots and Elizabeth I with two famous movie stars, Marina Rudd (played by Taylor) and Lola Brewster (played by Novak).
The two actresses are old rivals who hate each other. Marina, who is making a much heralded comeback after a prolonged "illness" and retirement, when she, in reality, has had a nervous breakdown, and her husband, Jason Rudd (played by Hudson), who is directing the movie they are making, arrive with their entourage. When she learns that Lola will be in the movie as well, she becomes enraged and vents her anger.
Lola and her husband, Martin N. "Marty" Fenn (played by Curtis), who is producing the movie they are making, then arrive. Excitement runs high in St. Mary Mead, as the locals have been invited to a reception held by the movie company in a manor house, Gossington Hall, to meet the celebrities. Lola and Marina come face to face at the reception and exchange some potent and comical insults, nasty one-liners, as they smile and pose for the cameras. The two square off in a series of hilarious and cleverly written and performed cat-fights throughout the movie.
Marina however, has been receiving anonymous death threats. After her initial exchange with Lola at the reception, she is cornered by a gushing, devoted fan, Heather Babcock (played by Maureen Bennett), who bores her with a long and detailed story about having actually met Marina in person during World War II. After recounting the meeting they had all those years ago, when she arose from her sickbed to go and meet the glamorous star, Babcock drinks a cocktail that was made for Marina and drops dead from poisoning.
The incident is unfortunate for Marina's mental state, and she is beside herself. Everyone is certain she was the intended murder victim. Once filming begins on the movie, she discovers that apart from threatening notes made up of newspaper clippings, her cup of coffee on the set has also been spiked with poison, sending her into fits of terror.
The police detective from Scotland Yard investigating the case, Inspector Debmot Craddock (played by Fox), is baffled as he tries to uncover who is behind the attempt on the life of the actress and the subsequent murder of the innocent woman. The suspected are Ella Zielinsky (played by Chaplin), Jason's production assistant who is secretly having an affair with him and would like Marina out of the way, and the hotheaded actress Lola Brewster.
Inspector Craddock asks his aunt, the renowned amateur detective Miss Jane Marple, who injured her foot at the reception and is confined to her home, for assistance. The main suspect, Zielinsky, is then killed by a lethal nose spray after going to a pay phone in the village, where she called the murderer and threatened to expose them. Miss Marple, now back on her feet, visits Gossington Hall, where Marina and Jason are staying, and views where Babcock's death occurred. Working from information received from her cleaning woman, Cherry Baker (played by Wendy Morgan), who was working as a waitress the day of the murder, the determined elderly sleuth begins to piece together the events of the fatal reception and solves the mystery. By the time she has collected all the evidence to indicate who committed the crime, however, another death occurs at Gossington Hall, which sadly closes the case on who the murderer in St. Mary Mead actually is: Marina Rudd, who has apparently committed suicide.
In the film's denouement, Miss Marple explains the murders that have occurred. Heather Babcock's story was Marina's initial motive. Ms. Babcock suffered from German measles — a rather harmless disease to most adults, but problematic for a pregnant woman. Heather Babcock innocently infected Marina when she met her during World War Two. Marina was pregnant at the time; the disease caused her child to be born with mental retardation. Upon hearing Heather cheerfully tell this story, Marina was overcome with rage and poisoned her without thinking. She then spread the idea that she was the intended victim, delivering the death threats and poisoning her own coffee. Ella, who made phone calls to various suspects from the pay phone, accidentally guessed correctly, prompting Marina to murder her. As Marina is now dead, she will not be brought to justice. Jason, her devoted husband, confesses to Miss Marple that he actually administered the dosage of poison to save her from prosecution. However, Marina didn't touch the hot chocolate he made for her and rather poisoned herself.
Agatha Christie's inspiration for the motive may have come from an incident in the life of actress Gene Tierney. In June 1943, while pregnant with her first daughter, Tierney contracted German measles during her only appearance at the Hollywood Canteen. Because of Tierney's illness, her daughter was born deaf, partially blind with cataracts and was severely developmentally disabled. Some time after the tragedy surrounding her daughter's birth, Tierney learned from a fan who approached her for an autograph at a tennis party that the woman (who was then a member of the women's branch of the Marine Corps) had sneaked out of quarantine while sick with German measles to meet Tierney at her only Hollywood Canteen appearance. In her autobiography, Tierney related that after the woman had recounted her story, she just stared at her silently, then turned and walked away. She wrote, "After that I didn't care whether ever again I was anyone's favorite actress." Biographers have theorized that Agatha Christie used this real-life tragedy as the basis of her plot for The Mirror Crack'd from Side to Side. The incident, as well as the circumstances under which the information was imparted to the actress, is repeated almost verbatim in Christie's story. Tierney's tragedy had been well-publicized for years previously.