In the novel N or M?, Mrs Elizabeth Cayley is staying with her husband Alfred Cayley at Sans Souci in Leahampton. She appears entirely devoted to caring for her invalid husband who is constantly complaining, asking for mufflers and rugs and wanting to be repositioned. The other guests at the hotel consider her a nitwit and vacuous.
In their search for a pair of spies, Tommy and Tuppence do not rule out Mrs Cayley as a plausible suspect. As Tommy observes, nitwittishness can be assumed. Tuppence thinks Mrs Cayley really does overdo or overact her part. "There can't be many women quite as idiotic as she seems." Against that, Tommy notes that he has noticed that "being a devoted wife saps the intellect".
After Tuppence asks Mr Cayley to tell her about his views on Germany, she sees a curious expression on Mrs Cayley's face. She is unsure whether it is wifely jealousy at her husband's attention being given to another woman, or dissatisfaction that Mr Cayley is being "too outspoken in his political views".