A. J. Cronin's 1937 novel, The Citadel, was a frontal attack on the British medical establishment of its day, conjuring the Shavian quote that "all professions are conspiracies against the laity."

The protagonist, Andrew Manson, is a young Scottish medical school graduate who takes a position as a doctor's assistant in a poverty-stricken Welsh mining town in the early 1920s. He soon realizes that he's been hired to shoulder the entire workload of an incompetent physician suffering from hemiplegia, but he does receive some much-needed guidance from an alcoholic but skillful ex-surgeon, Phillip Denny. When a typhoid epidemic cuts a lethal swath through the town's populace, the two doctors join forces to locate and blow up the sewer that has been its source.

Manson is constantly fighting an uphill battle, not only against the medical superstitions of the uneducated townspeople, but also the ignorance, incompetence, and greed of the medical professionals he encounters. When he is finally able to obtain a new job in the nearby town of Aberalaw, he marries Christine, the idealistic schoolteacher who he has been seeing, and they leave together.

In his new position, Manson is able to do productive research on the relationship between coal-dust inhalation and the silicosis which plagues the miners. Yet here, too, he runs afoul of a local anti-vivisectionist faction due to his reliance on using animals in his experiments. His next career move finds him working with Whitehall's Coal and Metalliferous Mines Fatigue Board, where he is shunted into the meaningless projects of whimsical bureaucrats, whose indolence he despises.

Exhausted and frustrated by these failed attempts to make a difference in the life of his patients and in his medical research, Manson buys a modest private practice on the outskirts of London. While socializing with Freddie Hampson, an old friend from medical school, he learns of the existence of so-called "Harley Street" doctors, who earn a lucrative income from essentially hand-holding wealthy "patients," particularly women, who have no real ailments.

Manson soon expands his clientele to practice exactly this type of medicine. Seduced by the steady stream of money and adulation it brings, he ends up endorsing the same quack remedies, needless operations, and stays in over-priced clinics that are the norm on Harley St. But, as time goes on, he is revolted by the spectacle of endless waste and its dramatic contrast with the suffering and poverty he has witnessed. He comes to loathe his clients and, even moreso, himself. His abandonment of his former idealism has also put a strain on his marriage, although the long-suffering Christine is rarely vocal about her unhappiness.

One day he is utterly stunned when a "Harley St." surgeon kills one of his patients by botching a simple operation, and Manson suddenly realizes he can no longer continue in this life. This breakthrough allows him to reconnect emotionally with his wife, who is grateful that he has finally come to his senses. But their reunion is short-lived; not long afterward, Christine is hit by a bus and killed. Manson is consumed with grief, barely able to function. But his friend and former colleague Phillip Denny returned from a sojourn in Mexico and helps him through these months of anguish. After an episode in which Manson is forced to defend his action in allowing a non-physician to successfully treat the thoracic condition of one patient with an innovative technique, he and Denny leave London to open an experimental medical clinic in the Midlands.