While visiting an elderly aunt in a nursing home, Tuppence Beresford becomes unsettled by a cryptic remark about a child hidden behind a fireplace, sparking a dangerous investigation. Tuppence and her husband, Tommy, find themselves drawn into a web of past secrets and present-day malice as they attempt to decipher the meaning behind the strange comment. The narrative follows the duo as they navigate the insular world of a retirement village and the surrounding countryside, facing opposition from those desperate to keep the truth buried. The story unfolds through a traditional third-person perspective, emphasizing the couple's domestic dynamic and their contrasting approaches to sleuthing.

This novel is widely recognized as a late-career entry in the Tommy and Tuppence series, showcasing Christie's ability to blend cozy mystery elements with darker, more unsettling psychological undertones. Readers often note the shift in tone compared to the earlier, more lighthearted adventures of the pair, as the narrative leans into the eerie atmosphere of the nursing home setting. The mystery is frequently praised for its intricate puzzle-box structure, which requires the protagonists to piece together fragments of history that have long been forgotten by others. While some critics argue the pacing is more deliberate than her earlier works, the consensus remains that the character development of the aging couple provides a compelling anchor for the suspenseful plot.