Based on a true story, a bright young man who hasn't the patience for the normal way of advancement finds that people rarely question you if your papers are in order. He becomes a marine, a monk, a surgeon onboard a Canadian Warship, and a prison warden.

On New England's Haven Island, Coast Guard agents arrest Ferdinand Waldo Demara, Jr. for impersonating numerous professionals, including his current alias, that of an elementary school teacher. Demara is handcuffed to his berth and escorted to the mainland, but all the while his concern resides mainly with the welfare of the students he has left behind. During the journey, Demara recalls the events that have brought him to his present predicament: As a child, Demara's grandiose ideas and bright curiosity are encouraged by his father, whose subsequent professional failures distress the young boy. Despite the sober counsel of local priest Father Devlin to "face facts" and settle for less, Demara continues to enjoy grandiose dreams. As a young adult, he quits high school in order to teach himself, not realizing that this will impede him when he joins the Army and attempts to become an officer. Prohibited from officers' training school, Demara then falsifies his credentials to match those of an accomplished stranger, Yale graduate Robert Lloyd Gilbert, and is immediately commissioned by the Marines. They inform him, however, that his papers will be sent to the FBI, and in order to avoid arrest, Demara fakes his own suicide and swindles his way into a Trappist monk monastery. There, Demara struggles with the ascetic rules of silence, piousness and, above all, lack of food. Although he strives resolutely, he is soon expelled. Outside the monastery, a farmer offers Demara a ride to town and his first taste of moonshine, which immediately lands him in jail. Soon discovered there by the FBI, he is sentenced to six years in a military disciplinary barracks, but Demara's charm and intelligence so impress Warden Ben W. Stone that he releases the young man after innocently providing him with detailed information about his own background. Demara then pretends to be Stone in order to gain work at a Texas prison, where Warden Chandler is enough impressed by his credentials and attitude to assign him to the maximum security cell block. There, Demara bravely eschews guard Lt. R. C. Brown's method of vicious control in favor of humane treatment, soon winning the respect and trust of the hardened inmates. He offers each man release from maximum security in return for a promise of contrition, but the men's brutal code of honor prevents them from accepting, which would be a tacit admission of weakness. While Demara's success grows, he also attracts the attention of Chandler's vivacious daughter Eulalie. One day, inmate Clifford Thompson attacks Brown with a knife. Demara restrains Brown from beating Cliff and instead challenges the inmate to a fair fight. Although Cliff is stronger than Demara, he allows the assistant warden to triumph, knowing that this way he will save face and Demara will treat him fairly. Soon after, Cliff is the first to break "the code" by asking to leave maximum security. Demara earns a promotion, but is then approached by a new inmate, Barney, who served with Demara in military prison and now threatens blackmail. With no other choice, Demara flees Texas and returns home, where he confesses his many personas to Father Devlin. Unable to convince Demara to turn himself in, Devlin prays for him to fall in love, an event that will force him to become honest. Undeterred, Demara joins the Royal Canadian Navy with the phony credentials of esteemed physician Dr. Joseph C. Mornay. By reading voraciously and following the previous doctor's orders, Demara manages to gain medical knowledge. One day, he spots nurse Catherine Lacey and falls in love at first sight. Although he tries to tell her the truth about his background, she refuses to listen, preferring the romance of the unknown. His love for Catherine, as Devlin has predicted, spurs Demara to want to clear his name, and to this end, he transfers to Korea and promises Catherine that once he "wipes away his guilt," he will return for her. Demara boards a Naval ship where the captain needs an emergency dental procedure, and although he administers an excess of anesthetic, the captain enjoys his long sleep and considers Demara a professional. When a recently attacked Korean ship approaches, bearing dozens of men in critical condition, Demara works day and night to treat them all, performing several miraculous operations. Months later, Demara has opened a hospital in Chinnampo and is earning worldwide press as "The Miracle Doctor." Unfortunately, the real Dr. Mornay soon sees Demara's photo in the newspaper and exposes him as a fraud. The news about "The Great Impostor" spreads throughout the world, reaching each of his old contacts and lovers, including Catherine. As Demara faces a Canadian board of inquiry to determine if he should be court-martialed, his lawyer informs him that each of his former employers has volunteered to speak in his behalf. Demara is pleased until he hears that Catherine is coming to visit and, ashamed, insists on addressing the board. There, he convinces them to spare themselves further embarrassment by allowing him to leave the country quietly. He awakens in the present, on the Coast Guard boat, unaware that Devlin and Catherine await him on shore with the news that she still wants to marry him and the Haven Island board of education insists that he return to them. By the time the boat docks, Demara has escaped out the port hole. Years later, the FBI hopes that their new expert agent, Sgt. Wilkerson, will crack the current search for Demara. What they fail to realize is that Wilkerson is, in fact, Demara in yet another disguise.