In his story "The Luncheon" Maugham focuses on depicting a single character, as he does so often in his short stories. The woman the narrator takes to luncheon at one of the most expensive restaurants in Paris is exploiting him mercilessly by pretending an interest in his writing which she probably doesn't really feel. At the same time, Maugham, who was twenty years younger at the time of the luncheon he is describing, is pretending to be urbane, gallant, and sophisticated. He has to keep up a smiling, insouciant facade while inwardly he is suffering agonies when his guest, who claims she never eats anything for luncheon, orders some of the most expensive things Foyot's has to offer, including salmon, caviare, and champagne. The fact that she orders everything a la carte probably makes her feel she is just nibbling tidbits.
Part of the irony derives from the fact that the narrator has never dared to go to Foyot's by himself because he is living on a very small income; and further, he has to watch his voracious guest devouring gourmet comestibles while he has to pretend he only wants a mutton chop and a glass of water.