At the end, Miss Marple quotes the last three lines:
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.
Francis Iles (Anthony Berkeley Cox) was somewhat muted in his praise in his review in The Guardian of December 7, 1962 when he said, "she has of course thought up one more brilliant little peg on which to hang her plot, but the chief interest to me of The Mirror Crack'd from Side to Side was the shrewd exposition of what makes a female film star tick the way she does tick. And though one could accept a single coincidence concerning that married couple, the second and quite wildly improbable one tends to destroy faith in the story – still more so since it leads nowhere at all.
Maurice Richardson of The Observer of November 11, 1962 summed up, "A moderate Christie; bit diffuse and not so taut as some; still fairly easy to read, though.
Robert Barnard: "The last of the true English village mysteries in Christie's output, and one of the best of her later books. Film milieu superimposed on the familiar St Mary Mead background. Like most Marples this is not rich in clueing, but the changes in village life and class structure since the war are detailed in a knowledgeable and fairly sympathetic way.
In June 1943, while pregnant with her first child, Tierney came down with German measles, contracted during her only appearance at the Hollywood Canteen. The baby, Daria, was born prematurely, weighing only 3 pounds, 2 ounces, and requiring a total blood transfusion. The infant was also deaf, partially blind with cataracts, and severely retarded and ultimately had to be institutionalized.
Some time after, Tierney learned from a fan who approached her for an autograph at a garden party that the woman, who had been a member of the women's branch of the Marine Corps, had sneaked out of quarantine while sick with German measles to meet her at her only Hollywood Canteen appearance. This incident, as well as the circumstances under which the information was imparted to the actress, is repeated almost verbatim in the story.
The novel was adapted for a 1980 feature film with Angela Lansbury in the role of Miss Marple. Co-stars were Elizabeth Taylor and Kim Novak and the film was released under the shorter US title of the book.
The novel was serialised in the Star Weekly Novel, a Toronto newspaper supplement, in two abridged instalments from March 9 to March 16, 1963 under the title The Mirror Crack'd with each issue containing a cover illustration by Gerry Sevier.