Ignatius Jacques Reilly is an overweight and unemployed thirty-year-old with a degree in Medieval History who lives with his mother, Irene Reilly. He utterly loathes the world around him, 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 the latter produce identification. Affronted and outraged by Mancuso's unwarranted zeal and officious manner, 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, denouncing Officer Mancuso and the police as communists. In the resulting uproar, Reilly and his embarrassed mother escape, taking refuge in a bar in case Officer Mancuso is still in hot pursuit.
In the bar, Mrs. Reilly then drinks too much. As a result, she crashes her car. The fallout for the accident totals $1020, a sizable amount of money in early 1960s New Orleans. Ignatius is forced to work for the first time in many years in order to help his mother pay for the accident.
What follows is a series of adventures that introduce an assorted cast of characters and their interactions with each other due to, or with, Ignatius as he moves from low wage job to job. Throughout the novel, Ignatius obsesses over his wardrobe, verbally abuses his mother, and frequents movie theaters only to yell and condemn the actors and actresses on screen. The novel explores the psyche of a man who is debilitated every time he is stressed out due to a rare stomach condition and an adversarial relationship possibly disguised as flirtation with the politically liberal advocate Myrna Minkoff, his only friend from college.
