In the novel N or M?, Alfred Cayley and his wife are staying at Sans Souci in Leahampton. Cayley is a middle-aged invalid or pretends to be and is constantly ministered to by his attentive wife who constantly plies him with mufflers and rugs and positions him in appropriate places in the hotel, out of the sun, or out of the draught. Major Bletchley, a fellow guest, calls him "a walking chemist's shop. Talks of nothing but his health and the treatment he's tried and the drugs he's taking". But he is equally ready to rattle off, with statistics and figures about how prepared Germany is for war, with her scientific methods of substituting for raw materials. He disagrees with the other guests who think the war will end soon. He believes it will last for another six years.
Mr Cayley expresses the opinion that Betty Sprot should be kept quiet, and that it is a "foolish modern spirit" to let children do as they please. Apparently, his doctor has said that it is important for him to get as much sleep as possible. However, when Tuppence suggests that he go to a nursing home, he says that such places are "ruinously expensive", and that there is "a suggestion of illness" in the atmosphere which "reacts unfavourably" on his subconscious.
According to Mrs Cayley, the doctor had suggested that a guesthouse would be better than taking a furnished house, because Mr Cayley would be less likely to brood, and "would be stimulated by exchanging ideas with other people". Tuppence is of the opinion that Mr Cayley;s method of exchanging ideas is to tell the listener about his ailments and symptoms.
Mr Cayley is an admirer of the Nazi system, and he hints that it would have been better for England and Germany to have allied themselves against the rest of Europe.
One evening, when Mrs Cayley is playing bridge with the other female residents of Sans Souci, Mr Cayley goes for a walk around the gerden. He enjoys being the "neglected invalid", "coughing in a sepulchral manner" and "shivering dramatically". He tells Mrs Cayley that even if he has caught a severe chill, it does not really matter, as there is a war on.