Dr. Alan Stone, a psychiatrist, begins working at Ypsilanti State Hospital in Michigan in the late 1950s. He becomes intrigued by three patients—Joseph, Leon, and Clyde—each of whom believes he is Jesus Christ. Fascinated by their shared delusion, Dr. Stone decides to conduct an experiment by placing all three men in the same ward with the hope that confronting the others' claims might lead them to question their own beliefs. Initially, this approach is met with skepticism from his colleagues and resistance from the patients themselves.

As Dr. Stone engages deeply with Joseph, Leon, and Clyde, he employs empathy and unconventional methods to understand their psyches better. His goal is not just to observe but to foster a breakthrough that could change psychiatric care. Tensions rise as the men interact with one another, each vying to maintain his identity while grappling with the contradictions presented by the others' existence. Throughout this process, Dr. Stone also contends with his own challenges and doubts about the ethical implications of his experiment. This complex interaction leads to moments of profound insight and raises questions about faith, identity, and what it means to be sane.