As a Congressional panel in Washington, D.C. launches an investigation into the infamous "Brubaker Case," a newscaster recounts how this "dark chapter in human history" came about: Gus Brubaker, a good-natured "schnook" living in New Jersey, is browbeaten by his wife Marge to file for the GI insurance to which he is entitled for serving in World War II. When Gus goes to apply, the Army clerk is peeved to discover that he has been assigned two different serial numbers: one for 1941-1945, the period in which he served as a tail-gunner and was shot down and declared dead, and one in 1945, after he was rescued and declared to be alive. To insure consistency, the clerk records only Gus's first number. As a result of his application, Gus receives a letter notifying him to report to duty and is shipped to Shima, an obscure rocky island off the coast of Japan. There, he meets a motley band of soldiers under the command of the eccentric Capt. Charles Stark. When Gus asks for a discharge, Stark cackles hysterically and points to a filing cabinet stuffed with discharge requests. The base has been long forgotten by the Air Force, and hence the men, bored to distraction, wile away their days organizing nature walks and badminton classes. While dunking some beer cans into the river one day, Gus meets Ume Tanaka, a beautiful young villager, who tells him that the water is hot, fed by springs flowing from the mountain above. Recognizing the potential of the springs, Gus suggests to Stark that they erect a hotel on the spot. When Stark questions his sanity, Gus points out that they can use government surplus and scrap lying about the island for building materials, and that the hotel would be a magnet for military and government personnel on leave. After Gus agrees to supervise the construction and sign the corporation papers, he and Doc Dave Farrington, the sage, base physician, go to make friends with the hostile villagers on the island. Ume, the mayor's daughter, helps them forge a cordial business relationship with the islanders. Soon after, the officious Lt. Nora McKay arrives at the base, summoned by Doc to lend a woman's touch to the hotel. Nora's presence prompts the men to doff their Hawaiian shirts and don their uniforms. As construction proceeds, Nora suggests using old parachutes for drapes and tents for cabanas. One day, a group of village girls arrives and informs the astonished Gus that they have been purchased to work at the hotel and he is now their "Papa-San." To publicize the hotel, Doc decides to trick his friend Joab Martinson, a medical journalist, into thinking that the springs have miraculous curative powers, offering the promise of eternal youth. Traveling with Gus to a medical conference in Tokyo, where Joab is billed as one of the featured speakers, Doc hands Joab a vial containing the "miracle" water, and when Joab asks for more, Doc suggests giving the hotel a plug in his column in exchange for an unlimited supply of the water. After Joab anoints the hotel as a "Fountain of Youth," military personnel flock to the island. Among them is Col. Archie Hollingsworth who, unaware that his underlings are running the place, asks for a set of hotel drapes for his living room. Meanwhile, Stark has fallen in love with Nora, but when he proposes, she accuses him of being a "sky jockey" and insists that he settle down and take a desk job. Soon after, Joab arrives, and when he drunkenly accosts the hotel maids, Stark pushes him into the swimming pool. In revenge, Joab writes an expose about the "sin city" of the Orient. Joab's story provokes a Congressional investigation, and a group of Senators, led by Gen. Weigang and Col. Hollingsworth, fly to the hotel to find the company clerk delivering room service. After the committee learns that Gus is the legal owner of the hotel, which was built from Army scrap, and that the female staff calls him Papa-San, an official hearing is convened and Gus is court-martialed and dubbed the "Vice King of the Orient." When Stark attempts to defend Gus, Hollingsworth orders him to ship out and Nora urges him follow orders. After Gus picks Doc to be his lawyer, they discover that Gus has been assigned the wrong serial number, the one belonging to the deceased Gus Brubaker. Although Doc argues that Gus's efforts have improved the morale of the base and the villagers, his testimony is contested by Hollingsworth. As the trial continues, Stark, returning from exile, buzzes the courtroom with his jet and then parachutes down to testify on Gus's behalf. Doc then blackmails Hollingsworth into letting Stark speak by reminding him of the drapes he illegally procured from the hotel. Afterward, Stark confronts Nora and angrily declares that she must accept him as is, and she meekly agrees. At the hearing's conclusion, Doc argues for a mistrial on the grounds that the serial number of the soldier they are trying belongs to a dead man. After Doc proposes giving the hotel to the villagers, Gus is found not guilty. In gratitude, the islanders carve a stone monument of Gus's profile, and soon after, Gus boards a boat bound for home.