The Korean War during the 1950s. The 4077th Mobile Army Surgical Hospital known as MASH sits just a few miles from the front line where the fighting is intense and seems to never stop producing patients in need of medical condition. The helicopters bringing in the wounded begin to outpace the ability of the surgeons to operate and so Col. Henry Blake, the Commanding Officer of this MASH unit, requests additional help. That help arrives in the forms of Duke Forrest and Hawkeye Pierce.
Both the surgeons were drafted into the Army and so they express a more outlandish disrespect for military etiquette and regulations than career Army doctors. This flippancy toward the abomination of trying to institute codes of conduct in the middle of the obscenity of an utterly unnecessary war only intensifies as they struggle to maintain their own sanity amid the insanity of a ceaseless parade of young men forever crippled by the machinery of warfare. This maintenance is exhibited through a series of mostly unconnected episodic vignettes interspersed gallows humor in the operating room designed primarily to put soldiers back to together so they can be sent right back into action.
While Duke and Hawkeye both exhibit a lack of deference and esteem for the very concept of the institution of a military, their primary targets of disgust are fellow doctor Maj. Frank Burns and head nurse Maj. Margaret Houlihan. Both are intensely devoted to the idea that sanity in a war zone can only be accomplished by recognizing and following the very same rules and regulations and codes of conduct that Hawkeye and Duke view as the obstruction toward keeping sane. In addition, Maj. Burns annoys them with his pious and judgmental religious beliefs that he attempts to force upon a local boy who helps clean up around the camp. This annoyance grows into outright detestation upon the revelation that Burns’ alleged morality has been utterly compromised by a hypocritically adulterous affair with Houlihan. One night, they sneak a microphone into the tent where the two are making love and Margaret’s request for Frank to kiss her hot lips is not only heard by everyone else in the MASH unit, but becomes her nickname.
Having convinced Col. Blake to have Burns moved from their tent, Duke and Hawkeye greet a new arrival: a chest surgeon who is suspiciously silent on the subject of his background. The fact that he carries martini olives around in his oversized coat quickly wins their friendship and admiration, however. A few days later when some of the guys are tossing a football around in the camp, the new guy—who is known as Trapper John—makes a fantastic catch which stimulates Hawkeye to realize where he’s seen this guy before: he was a college football star before becoming a surgeon.
The friendship only deepens when Trapper punches Maj. Burns after witnessing Frank blame someone else for the death of a patient that died as a result of his own malpractice.
After witnessing Houlihan get taunted with her new nickname followed by a disrespectful display toward his own sexuality, Burns assaults Hawkeye in a manic attack. Col. Blake responds by ordering Maj. Burns to be fitted with a straight-jacket and he is later seen leaving the camp in a jeep.
A few days after Burns disappears, the MASH dentist, Cap. Waldowski, comes to Hawkeye in desperation after a bout of impotence. This is especially distressing to Waldowski because of his reputation as a sexual powerhouse. Waldowski becomes convinced that the cause of his sudden inability to perform up to snuff is latent homosexuality. Unable to face up to this truth, he decides to commit suicide. The other doctors prepare a “Last Supper” celebration before he takes a “black capsule” as the means of killing himself. The night actually ends with Lt. Dish, a pretty nurse, demonstrating to the Captain that he has absolutely nothing to worry about when it comes to his sexual prowess.
The surgeons take bets on whether Hot Lips Houlihan is a real blonde and set up a prank by which the tent in which she shower can be hiked up to briefly reveal her nudity to the entire camp, thus proving whether she is a real blonde or not by comparing the color of the hair on her head to the color of her pubic hair. An enraged Hot Lips runs to Maj. Blake’s tent to demand that Hawkeye and the others be punished or else she will resign her commission. Blake lets her know his preference would be her resignation.
Trapper John later is requested to fly to Japan to operate on the son of an American politician. He in turn requests that Hawkeye accompany him and they show up in the hospital dressed for golf with their clubs in two. They immediately start the surgery just so they can hit the golf course in time to play before dark. The anesthesiologist turns out to be an old buddy of Hawkeye who invites them to a brothel which doubles as a hospital for kids. While there, an emergency arises which causes them to transport the offspring of a soldier and prostitute to the military hospital. They are denied access by the head of the hospital who claims it isn’t there for the purpose of treating the natives. The two surgeons respond by sedating the man and taking pictures of his with the prostitute in order to blackmail him against taking action against them.
Gen. Hammond arrives at the MASH unit not long after Hawkeye and Trapper get back in order to investigate complains made by Hot Lips. The conversation is turned to football and plans are set for a football game to be played between members of the 4077th and Hammond’s own 325th. Hammond only agrees to the game on condition of a $5,000 wager placed on the outcome.
Col. Blake requests the transfer of an Oliver Harmon Jones to his MASH unit. Jones was once known as “Spearchucker” when he played for the Philadelphia Eagles. Knowing that the only chance they have to win the game and make big money off side bets is keeping Spearchucker’s true identity unknown, they train in secret and decide to hold off putting him into the game until the second half when they can negotiate better odds for themselves.
What they don’t know is that Hammond has his own ringer who is running roughshod over them. In order to take stop him, they decide to take him out of competition completely by furtively injecting a sedative into him after a tackle. With their ringer out of the game, Spearchucker takes the field and the 4077th emerges victorious.
After a brief lull, it’s back to the operating room as usual until Duke and Hawkeye are interrupted during a procedure with news that their discharge orders have come through. Soon afterward, they leave the camp in the same stolen jeep in which they arrived.