In the novel N or M?, Commander Haydock is a retired Naval officer. His home is called Smugglers' Rest and is located next to the hotel Sans Souci in Leahampton on the English south coast. According to Major Bletchley, Commander Haydock is the local ARP warden.
Commander Haydock is described as being a "big hearty man with a weather-beaten face, intensely blue eyes, and a habit of shouting most of his remarks".
Commander Haydock's pet theory is that sooner or later the Germans will attempt a landing at Leahampton, because it is such an unimportant place, and so is not heavily guarded. He remarks that there is not even an antiaircraft gun in the place, which he feels is disgraceful. He also expresses the opinion that there is no organisation or proper coordination in the defences of Leahampton, and believes that he ought to be in charge. He is determined to take over from Colonel Andrews if possible.
Commander Haydock tells Tommy and Major Bletchley about the history of Smuggler's Rest, including how it had previously been bought by a man named Hahn, whom he had suspected of being a German spy.
When Betty Sprot is kidnapped, Major Bletchley suggests asking Commander Haydock for help in going after her, because he has a car. Commander Haydock is a "good organiser and a quick worker". He quickly fills the car with petrol, and provides a map of the district, as well as a larger scale map of Leahampton. He also brings a pair of field glasses and a service revolver.
Commander Haydock later invites Tommy to Smuggler's Rest for drinks. He loses his temper at Appledore, when he accidentally spills a glass of crême de menthe on Tommy.
It is later revealed that Commander Haydock is the German agent N, and has a secret transmitting wireless device hidden in his bathroom. After Tommy discovers this, he realises that Commander Haydock's face, which he had previously thought to be the face of a "hearty Englishman", is really the face of "a bad-tempered overbearing Prussian officer".
Commander Haydock and his associates kidnap Tommy, planning to keep him alive until a boat arrives to take him away, so that his disappearance cannot be connected with Smugglers Rest.
Commander Haydock later meets with Tuppence, under the name of Dr Binion. He deamnds to know where "it" is, and holds her at gunpoint, giving her three minutes to tell him where she has hidden "it". He is so intent on watching Tuppence that he does not notice the door opening behind her, and he is shot by one of Mr Grant's men. It is revealed that the item Commander Haydock had been asking Tuppence for was a book which contained the names of prominent people who were pledged to assist an invasion of England.