“You are disappointed,” Ausable said wheezily over his shoulder. “You were told that I was a secret agent, a spy, dealing in espionage and danger. You wished to meet me because you are a writer, young and romantic. You envisioned mysterious figures in the night, the crack of pistols, drugs in the wine.”

     “Instead, you have spent a dull evening in a French music hall with a sloppy fat man who, instead of having messages slipped into his hand by dark-eyed beauties, gets only a prosaic telephone call making an appointment in his room. You have been bored!” The fat man chuckled to himself as he unlocked the door of his room and stood aside to let his frustrated guest enter.
Ausable then observed that Fowler was disappointed after meeting him. Because of some breathing problems, he made a high, rough noise over his shoulder. Ausable then explained why he said it in detail. He told Fowler that he might have heard from someone that Ausable was a secret agent dealing with espionage and danger. The term "espionage" refers to the practice of spying or the use of spies, often by the government, to gather political and military information. After telling that, Ausable said that Fowler had shown an interest in meeting him because he was a writer who looked very young and romantic.

Ausable then said that Fowler might have imagined encountering several strange people in the middle of the night. Also, he visualised seeing shadowy creatures in the night, hearing gunshots, and using narcotics in the wine. On the contrary, Fowler had spent a dull evening at a French music hall with a chubby fat man.
Imagining encounter

Ausable then stated that Fowler would have expected Ausable to get communications from the dark-eyed beauties. Here the term "dark-eyed beauties" refer to beautiful ladies with dark eyes. In this context, dark-eyed beauties were used to describe the type of ladies Fowler hoped to see around Ausable. Fowler might have such thought because he was a romantic and young writer; he would expect how a secret agent would be. On the contrary, Ausable used to get ordinary telephonic calls, and people would phone him to schedule an appointment to meet him in his room. After telling that, Ausable said that Fowler might have felt bored of seeing him. He made the statement because Fowler had thought of meeting a different personality. But while seeing the fat man, Fowler would have felt bored.

Ausable then laughed quietly to himself when he opened his room. He stepped aside to allow his frustrated friend to enter his room. Ausable would have referred to Fowler as a "frustrated friend" because whatever Fowler had expected had not occurred. Fowler might have felt irritated on seeing the ambience of the room and him.
Meanings of the difficult words:
ShoulderThe upper joint of each of a person's arms and the part of the body between this and the neck
Espionage The practice of spying or the use of spies, often by governments to gather political and military information
Prosaic Ordinary and not especially interesting or unusual
Frustrate Feeling annoyed or less confident because you cannot achieve what you want
ChuckleLaugh quietly or inwardly
National Council of Educational Research and Training (2007). The Midnight Visitor- Robert Arthur (pp. 14-19). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.