Ignatius Jacques Reilly is an overweight and unemployed 30-year-old with a master's degree in Medieval History. He lives with his mother, Irene Reilly. He loathes the world, which he feels has lost the values of geometry and theology. One afternoon Reilly's mother drives him "downtown in the old Plymouth, and while she was at the doctor's seeing about her arthritis, Ignatius had bought some sheet music at Werlein's for his trumpet and a new string for his lute." While Reilly waits for his mother, Officer Angelo Mancuso approaches Reilly and demands that Ignatius produce identification. Affronted and outraged by Mancuso's zeal and officiousness, Reilly protests his innocence to the crowd while denouncing the city's vices and the graft of the local police. An elderly man, Claude Robichaux, takes Reilly's side and denounces Mancuso and the police as communists. In the resulting uproar, Mancuso arrests Robichaux while Reilly and his embarrassed mother escape and take refuge in a bar.
There, Mrs. Reilly drinks too much. On the way home, she crashes her car. The bills for the accident total $1,020, a sizable amount of money in early 1960s New Orleans and which would be a little over $10,000 in 2023. To help his mother pay for the accident, Ignatius must work for the first time in years.
What follows is a series of adventures that introduce characters and their interactions as Ignatius flounders from one low-wage job to another. Ignatius obsesses over his wardrobe, verbally abuses his mother, and frequents movie theaters, where he condemns the actors and actresses. The novel explores the psyche of a man who, every time he feels stress, observes the action of his pyloric valve. Reilly maintains an adversarial relationship, and possibly a flirtation, with the politically liberal Myrna Minkoff, his only friend from college.
