The book's title is derived from a lawyer who wins spectacular cases and winning huge sums in damages being called "a rainmaker". The book's protagonist dreams of becoming such a "rainmaker" - which comes true, but not exactly in the way he expected.

Rudy Baylor is about to graduate from Memphis State Law School. He secures a position with a Memphis law firm but loses the job when the firm is bought out by the large Memphis law firm Tinley Britt. As one of the few members of his class without a job lined up, a desperate Rudy is introduced to J. Lyman "Bruiser" Stone, a ruthless but successful ambulance chaser, who makes him an associate. To earn his fee, Rudy is required to hunt for potential clients at the local hospital and sign them up to personal injury lawsuits. He is introduced to Deck Shifflet, a less-than-ethical former insurance assessor who received a law degree but doesn't practice law, having failed to pass the bar exam six times.

Rudy signs two clients. One is his new elderly landlady, who needs a revised will drawn. The other is a poor family, Dot and Buddy Black, whose insurance bad faith case could be worth several million dollars in damages. With Stone's firm about to be raided by the police and the FBI, Rudy and Deck set up their own practice and file suit on behalf of the Blacks, whose leukemia-stricken son, Donny Ray, could have been saved by a bone marrow transplant for which his identical twin brother is a perfect match. The procedure should have been covered and paid for by their insurance carrier, Great Benefit Life Insurance, but the claim was instead denied.

Rudy, having just passed his bar exam, has never argued a case before a judge or jury. He now finds himself up against experienced and ruthless lawyers from Tinley Britt, headed by Leo F. Drummond. On his side, Rudy has several supporters and a sympathetic, newly-appointed judge. While preparing the case in the local hospital, he meets and later falls in love with Kelly Riker, a young battered wife recovering from injuries inflicted by her husband Cliff.