In the novel N or M?, Carl von Deinim is one of the guests at the hotel Sans Souci in Leahampton. He is of German origin.
Carl is a young man, and has fair hair and blue eyes. He is a research chemist, and works at a chemical research laboratory. According to Mr Grant, Carl's work at the laboratory has been excellent and helpful in immunising certain gases and in general decontamination experiments.
Carl's father was indiscreet, was arrested, and was sent to a concentration camp, where he died. His elder brothers were also sent to concentration camps. His mother died "in great distress of mind" a year before the events of the novel. Carl came to England as a refugee, and according to Mr Grant, had professed himself anxious to help England.
Tuppence sees Carl talking with Sheila Perenna, and overhears Sheila telling him that he must be careful. On another occasion, she sees him talking to Vanda Polonska. This seems to cast suspicion on Carl, but Tuppence hopes that he and Sheila "aren't in it", because she likes them.
Carl later tells Tuppence that it would be best for him "to end everything". He says that he fled from Germany because of injustice and cruelty, and to find freedom, and that he hates Nazi Germany, but he is still a German, and nothing can change that. He tells her that he is moved to fury when he hears of German cities being bombed and German soldiers dying, and when he hears Major Bletchley referring to Germans as "those swine". He also says that he wishes they would intern him, because that would be easier.
Tuppence tells Carl that he is doing work that is useful, not only to England but to humanity, and that anything that mitigates suffering and is constructive is worth doing. Carl takes her hand and kisses it, and tells her that what she says is true, and that he will have more fortitude.
Tommy later sees Carl go into Tuppence's room when she is not there, and he remains there for a quarter of an hour. This leads both Tommy and Tuppence to believe that Carl is an enemy agent.
Carl is later arrested, and his room at Sans Souci is searched. According to Mr Grant, a list of people to be approached as possible Fascist sympathisers is found in Carl's room. There is also a clever scheme of sabotage and a chemical process which would devastate food stocks if applied to fertilisers. Mr Grant says that Carl also had a supply of secret ink, which is "a pretty good clinching test". A shoelace had been steeped in secret ink, and could be soaked in water when the ink needed to be used.
It is later revealed that Carl is innocent, and that the things found in his room, including the shoelace dipped in secret ink, had been planted by M.
It is further revealed that he is not the real Carl von Deinim, and that he is in the Intelligence, and was selected for work in Germany as his grandmother was German.
The real Carl von Deinim had been his friend, whom he had known in England, and with whom he had renewed his friendship in Germany. The real Carl had decided to escape to England before the war started, and the authorities appeared to make it easy for him to leave Germany, despite his brothers and other relations being in concentration camps. When the real Carl died, his friend assumed his identity, travelled to England, and stayed at Sans Souci.
After his arrest and the discovery of the faked evidence, "Carl" says nothing, as he wants to see what will happen. He is eventually recognised as a member of the Intelligence, and is released.
At the end of the novel, "Carl" attends a party hosted by Tommy and Tuppence, where he dances with Sheila.