Kurt Müller is an agent with British Intelligence.
Eventually.
When we first encounter him, he is encountering a corpse. Rather an interesting introduction.
The year is 1940. German forces have just broken through all defenses in Belgium, the British were in full retreat to Dunkirk. Austria and most of both Poland and Czechoslavakia are under Hitler's control.
Müller is a young man recently out of college who has a thankless and tedious job in the signals department of the AbWehr, the German Military Intelligence. He was so assigned because of his knowledge of expertise in English, coming from having an Irish mother and having gotten his undergraduate degree from Trinity College not long before we meet him. Since most of his work just involves receiving Morse code, his language skill is for the most part wasted.
That is, until his uncle made a comment and things changed quickly. His uncle was not really an uncle so much as having a sister who was married to Müller's father's brother. Still, it is a contact and when the 'uncle' invited him to a weekend party, he went and happened to mention his job and suddenly he is transferred to a more interesting job of actually breaking codes with the Enigma device. This 'uncle' was Reinhard Heydrich, notorious head of the Reich Main Security Office, the bosses of the Gestapo.
Müller's future might have been a promising one if he had wanted to play off his influence of his 'uncle', which he did not want to. And if he ignored his conscience, which he was unable to. What he would learn would drive him to the other side, first through the Resistance and then later with British Intelligence.