Helen Eustis has a gift for portraying characters in various states of mental anxiety, ranging from the normal through the highly neurotic to the psychotic. In the post-World War II era, she helped introduce into crime fiction a new quality of realism and sophistication in the portrayal of both the villain and the victim—the guilty and the innocent—which foreshadowed the development of the psychological plot. Her stories show how people placed in threatening circumstances react in bizarre, often incriminating ways.
Influenced by Edgar Allan Poe and Fyodor Dostoevski, to whom she alludes in The Horizontal Man (1946), Eustis adds a note of clinical realism to the gothic terrors experienced by her characters by explaining their behavior in terms of the pathology of the criminally insane. In The Horizontal Man, Eustis combined knowledge of abnormal psychology with mastery of the genre’s least-likely-suspect convention to produce a tour de force rivaling Agatha Christie’s The Murder of Roger Ackroyd (1926).