In 1951, the 4077th Mobile Army Surgical Hospital in South Korea is assigned two new surgeons, “Hawkeye” Pierce and “Duke” Forrest, who arrive in a stolen Army Jeep. They are insubordinate, womanizing, mischievous rule-breakers, but they soon prove to be excellent combat surgeons. Other characters already stationed at the camp include bumbling commanding officer Henry Blake, his hyper-competent chief clerk Radar O’Reilly, dentist Walter “Painless Pole” Waldowski, the incompetent and pompous surgeon Frank Burns, and the contemplative Chaplain Father Mulcahy.
The main characters in the camp divide into two factions. Irritated by Burns’ religious fervor, Hawkeye and Duke get Blake to move him to another tent so newly arrived chest surgeon Trapper John McIntyre can move in. The three doctors (the “Swampmen”, after the nickname for their tent) have little respect for military protocol, having been drafted into the Army, and are prone to pranks, womanizing, and heavy drinking. Burns is a straitlaced military officer who wants everything done efficiently and by the book, as is Margaret Houlihan, who has been assigned to the 4077th as head nurse. The two bond over their respect for regulations and start a secret romance. With help from Radar, the Swampmen sneak a microphone into a tent where the couple are making love and broadcast their passion over the camp’s PA system, embarrassing them badly. The next morning, Hawkeye goads Burns into assaulting him, resulting in Burns’ removal from the camp for psychiatric evaluation. Later, the Swampmen humiliate Houlihan again by pulling up the walls of the shower tent while she is inside, exposing her naked body for all to see.
Painless, described variously as “the best-equipped dentist in the Army” and “the dental Don Juan of Detroit”, becomes depressed over an incident of impotence and announces his intent to commit suicide, believing that he has turned homosexual. The Swampmen agree to help him carry out the deed, staging a feast reminiscent of the Last Supper, arranging for Father Mulcahy to give Painless the last rites, and providing him with a “black capsule” (actually a sleeping pill) to speed him on his way. Hawkeye persuades the gorgeous Lieutenant “Dish” Schneider, who is being transferred back to the United States for discharge, to spend the night with Painless to allay his concern over his sexual preference. The next morning, Painless is his usual cheerful self, and a smiling Dish leaves camp in a helicopter to start her journey home.
Trapper and Hawkeye are sent to Japan on temporary duty to operate on a Congressman’s son. When they later perform an unauthorized operation on a local infant, they face disciplinary action from the hospital commander for misusing Army resources. Using staged photographs of him in bed with a prostitute, they blackmail him into keeping his mouth shut.
Following their return to camp, Blake and General Hammond organize a game of football between the 4077th and the 325th Evac Hospital and wager several thousand dollars on its outcome. At Hawkeye’s suggestion, Blake applies to have a specific neurosurgeon – Dr. Oliver Harmon “Spearchucker” Jones, a former professional football player for the San Francisco 49ers – transferred to the 4077th as a ringer. Blake bets half his money up front, keeping Jones out of the first half of the game. The 325th scores repeatedly and easily, even after the 4077th drugs one of their star players to incapacitate him, and Hammond confidently offers high odds against which Blake bets the rest of his money. Jones enters the second half, which quickly devolves into a free-for-all, and the 4077th gets the 325th’s second ringer thrown out of the game and wins with a final trick play.
Not long after the football game, Hawkeye and Duke get their discharge orders and begin their journey home – taking the same stolen Jeep in which they arrived.