Miss Marple investigates the murder of Heather Badcock, who consumed a poisoned cocktail apparently meant for American film actress Marina Gregg, Heather’s idol. As Marple investigates, she discovers dark secrets in Marina’s past, secrets which also link to other seemingly innocent citizens of St. Mary Mead.
At first, detecting the murderer is extremely difficult because there seems to be no motive. Because Marina Gregg had given Heather Badcock her own drink shortly after meeting her it is assumed that Marina Gregg must have been the intended victim.
Also Marina is much more famous and correspondingly more likely to be a target. However, it eventually becomes apparent that Marina herself poisoned the drink and intended to kill Heather Badcock. Discovering the murderer is complicated because the motive is so obscure.
It is known that when Heather Badcock encountered Marina Gregg at the party where she is murdered, she had told her her favourite anecdote about how, years before, she had been ill, but had sneaked out to meet Marina and get her autograph. A terrible expression appeared on Marina’s face as she heard this story, reminding a witness of the line from Tennyson’s poem.
Marina had always desperately wanted children but had found it difficult to conceive. However, after adopting three children, she had finally become pregnant. But when her baby was born it was found to be mentally retarded and was abandoned to a lifetime of institutions, leaving Marina emotionally scarred.
Miss Marple later deduces what Marina had instantly realised. Heather’s minor illness was German measles; she had infected Marina and caused the mental retardation, and effectively the ‘loss’, of her only child. Marina murdered Heather for revenge.
There can be little doubt that Christie used the real-life tragedy of American film and stage actress Gene Tierney as the basis of her plot.  Tierney described the event in her autobiography (Self-Portrait, New York: Wyden, 1979), but it had been well publicized for years previously.