Set during the early 1890s in a fashionable summer resort somewhere on the East Coast of the United States, the book is narrated by a Mr Twelvemough, a popular author of light fiction who has been selected to function as host to a visitor from the faraway island of Altruria called Mr Homos. Homos has come all the way to the United States to experience first-hand everyday life in the country which prides itself to represent democracy and equality, to see for himself how the principle that "all men are created equal" is being practiced.
However, due to Altruria's secluded existence very little is known about that state, so Twelvemough and his circle of acquaintances, all of whom are staying at the same hotel, are more eager to learn something about Altruria than to explain American life and institutions. To their dismay, it becomes gradually clear to everyone involved in the conversations with Mr Homos—who in the course of the novel becomes less and less reluctant to talk about his own country—that the United States is greatly lagging behind Altruria in practically every aspect of life, be it political, economical, cultural, or moral. Thus, in the novel the island state of Altruria serves as a foil to America, whose citizens, compared to Altrurians, appear selfish, obsessed with money, and emotionally imbalanced. Mainly, A Traveller from Altruria is a critique of unfettered capitalism and its consequences, and of the Gilded Age in particular.