Hunter S. Thompson chronicles a two-year period of the Hell's Angels, focusing on the San Francisco and Oakland chapters of the California-wide gang. He discusses their origins and background in general terms, their social significance and their unlikely rise to national fame through media exposure following several violent crimes. Thompson then details a motorcycle run to a particular destination. He followed the Hell's Angels and participated in their revelries. The text concludes with a discussion of the Hell's Angels in a larger social context, as well as a reflection on their nature and essential character.

The Hell's Angels are an infamous gang of motorcycle-riding outlaws. They are one of many such gangs and are based predominantly in Oakland and San Francisco, California, where the weather permits relative comfort while riding a motorcycle. Motorcycle gangs have their roots in post-World War II America, and were initially composed of dropouts and vagabonds who banded together for protection and camaraderie. The Hell's Angels were founded in 1950 near San Bernardino, California. They continued as another gang until roughly 1964, at which time they were composed of perhaps eight-five members scattered among several chapters. Ralph "Sonny" Barger headed the largest and best-organized chapter.

In 1964 the Hell's Angels were accused of a vicious gang rape of two underage girls. Although several members were arrested, the charges were later dropped. However national newspapers and magazines reported the incident. The resulting wave of infamy propelled the Hell's Angels to the forefront of motorcycle outlaw culture. An outraged citizenry demanded police action, and various law enforcement reports summarized the Hell's Angels' activities of the prior few years. Law enforcement began to blame a hugely disproportionate amount of criminal activity on the Hell's Angels, and repetitive news stories of fresh outrages swept through the national media.

By 1965 the Hell's Angels had emerged as the de facto leaders of all motorcycle gangs, and even began to transition into an apparently accepted power center within the liberal counter-cultural movement. Still, gang members were not educated intelligentsias rebelling against the war or politics; and within a few months their essentially criminal and vicious nature caused a permanent rift to develop between the motorcycle gangs and the liberal Berkeley crowd.

Hunter S. Thompson wrote a short newspaper article in 1964 about the Hell's Angels. He was later introduced to several Hell's Angels and was also offered a contract for writing a book on the gang. He began to spend more time with the motorcycle gang, eventually buying a motorcycle, spending many hours in the company of Hell's Angels, and even accompanying them on several road trips. Within several months he became an accepted fixture within the notoriously reporter-shy Hell's Angels. His resultant first-hand look at the Hell's Angels is chronicled in the book, which also includes personal experiences, as well as opinions about the Hell's Angels and the role the play within the greater American society.