En Corée, dans l'unité mobile de chirurgie hospitalière du 4077ème de l'armée (M.A.S.H), deux médecins de l'armée, Hawkeye Pierce et le trappeur John McIntyre, reçoivent des nouvelles intéressantes par courrier. Ho-John, leur garçon de maison coréen, a été accepté dans l'ancien collège de Hawkeye. Hawkeye et Trapper décident d'organiser une fête avec de la musique, de la danse et de l'alcool pour amasser des fonds pour le voyage en avion de Ho-John aux États-Unis.

Des voleurs dérobent un camion de fournitures médicales pour le 4077, les rendant ainsi dans le besoin désespéré d’hydrocortisone. Débrouillards, Hawkeye et Trapper rencontrent un spécialiste notoire du marché noir, dans l'espoir d'obtenir ces dernières. Quand ils réalisent qu'ils n'ont rien de raisonnable à échanger avec lui, ils prennent des mesures radicales pour obtenir ce dont ils ont besoin.

Frank Burns complains about Hawkeye Pierce's disrespect...and Henry appoints Hawkeye chief surgeon, to Burns's shock. The rest of the 4077th "coronates" Hawk while Frank and Hot Lips complain to General Barker. The General's visit provides him with a view of life at the 4077th M*A*S*H unit: camp hijinks, a poker game, and a surgery session. This shows him M*A*S*H has fun but gets the job done.

Sergeant Baker arrives at the camp with his Moose. Hawkeye decides to find a way of getting her away from Baker. He tries ordering him to release her, tries buying her, and then resorts to cheating at cards. He releases her but she won't go, so he tries to teach her how to be independent.

The 4077th is designated as the setting for the making of an army film on Mobile Army Surgical Hospital units. Hawkeye is chosen as the star while Margaret and Frank compose a screenplay. The Eye Of The Hawk objects to the piece of propaganda that filmmaker Lt. Bricker is producing and, having exposed the original film, reshoots a new one his way, starring himself as Groucho Marx-ish Yankee Doodle Doctor, and poking fun at glorifying doctors while concluding with a rather serious speech about the hell of war.

When Henry goes for some R&R, Hawkeye pretends to crack up so that Frank will let him and Trapper go on some R&R.

John Hodges, a chopper pilot referred to as The Cowboy because of his gun holster belt and cowboy hat, has been hit in the shoulder, and arrives at the 4077th. He is expecting a letter--he's worried his wife Jean at home is leaving him for another man ("She's probably off with some rodeo rider; she's a sucker for a 10-gallon hat!"). He wants to go home, but Henry refuses, stating Cowboy's wound isn't serious enough to merit a stateside ticket. Bad luck then follows Henry Blake like the seat of his pants: he gets shot at while golfing, his tent gets flattened by a driverless jeep, and the latrine explodes while he's inside. The Cowboy offers to fly Henry to Seoul and then threatens to shove him out! The letter finally arives for Cowboy, assuring he is loved. Hawkeye and Trapper uses the radio to convince The Cowboy to spare Henry's life and come down, at which they succeed.

Henry receives a citation for the camp achieving the best efficiency rating, and then General Hammond reassigns him to Tokyo. Frank then changes the camp to be more military, and he confiscates Hawkeye's and Trapper's still. They use forged passes to go to Tokyo to convince Henry to come back and end up pretending Radar is sick.

A rash of thefts breaks out in the camp. Missing pieces include Frank's silver picture frame, Margaret's hair brush, and Trapper's watch. The camp is searched and everything is found in Hawkeye's locker. Everyone thinks he did it. Hawkeye manages to announce to the camp that the items will be dusted for prints to identify the real thief, and catches Ho-Jon. He needed money to bring his family from the North, and to bribe border guards.

Hawkeye moves a wounded North Korean soldier into The Swamp, rather than let him be shipped out before he's stable. During the night he and Trapper play Dracula, and siphon off a pint of Frank's blood. The soldier then contracts hepatitis, so they have to test Frank without him knowing, and have to keep him away from Margaret and the patients.

Hawkeye writes home, describing Christmas in Korea: Radar ships a jeep home, a piece at a time; Henry gives the monthly lecture on sex, with the aid of figure A and figure B; Trapper helps deliver a calf; Klinger and Frank get into a fight, but Father Mulcahy smoothes things over; Hawkeye and Trapper sabotage Margaret's tent; Hawkeye flies to the front line dressed as Santa, to help a wounded soldier.

The nurses go to extremes lengths to find a date for Nurse Eddie - they won't go out with anyone until Eddie gets a date. The men draw straws, and Hawkeye is the big loser, especially after Eddie nearly kills him in a scene resembling teenage "mating" rituals.

Radar gets a Dear John recording from home. Hawkeye and Trapper try to set him up with a date, but fail. Radar is taken by a new nurse at the camp and she is into poetry and music, so they coach him. Margaret wants to stop the relationship, so Hawkeye and Trapper get between her and Frank until she relents. Radar's "Ahhhh, Bach!" and "That's highly significant," quotes win him the girl.

Hawkeye creates a fake doctor, Captain Jonathan S. Tuttle, to give supplies to the local orphans. Henry wants Tuttle to be officer of the day, so Hawkeye creates a fake personnel file, and all his back pay is given to the orphanage. When General Clayton wants to reward his generosity, Hawkeye is forced to invent a story about Tuttle jumping from a chopper without his parachute! Of course, Trapper's new friend, Captain Murdoch, obtained the fake dog tags and parachute...!

Hawkeye and Trapper operate on a famous Colonel - after discovering that he is particularly ruthless about sacrificing his men, they come up with a scheme to get him sent back to the states with a little unwitting help from Frank, Margaret, and a drunk Henry.

Frank throws his back out whilst spending the evening with Margaret, and ends up in traction. He promptly applies for the Purple Heart, having been 'technically' wounded at a frontline unit. Tommy Gillis, an old friend of Hawkeye's, is writing a book about the war, and pays him a visit. Later, Tommy is brought into the camp, seriously wounded, and Hawkeye can't save him. A 15-year-old kid is in the hospital to have his appendix out. He joined up to be a hero back home, but Hawkeye has him sent home, giving him Frank's purple heart.

Once again, Hawkeye writes home to his father, telling him of the latest gossip: the camp gets a new surgeon, who turns out to be a fake; Hawkeye bets he can walk into the mess tent naked for lunch, and no one will notice; Radar cheats on his final exam from the High School diploma company; Margaret rejects Franks advances and he gets drunk late into the night; the camp have a no talent night.

The camp suffers from the severe cold, except for Hawkeye who has received some long john's from his father. They get passed around from person to person, as a gift, a gambling stake, a trade, a bribe, stolen, given up to Father Mulcahy, who gives them to Henry, who returns them to Hawkeye as thanks for taking out his appendix.

The camp tunes into the Army/Navy football game, only to be shelled and have an unexploded bomb land in the middle of the compound. They ring around trying to identify the bomb, and the camp prepares for the worst. Hawkeye and Trapper are left the task of following instructions to disarm the bomb, which turns out to be full of propaganda leaflets from the CIA.

Hawkeye and Frank argue over Frank's surgical ability. Hawkeye performs a difficult operation and the patient does not recover, as he should. Hawkeye begins to doubt his ability and moves out of The Swamp. He decides to open up his patients again, and discovers a nick in the colon that even Frank admits anyone could have missed.

As usual Frank's normal drone of verbal abuse upsets Ginger, so Hawkeye puts his arm in a cast while he is asleep. Frank puts in for a transfer, and after a broadcast goes out of Frank telling Margaret he's leaving, she decides to leave as well. As a result, Col. Blake puts both Hawk and Trap on double post-op duty until he finds replacements for Majs. Burns and Houlihan.

Unwilling to lose their two favorite patsies, and to be worn to a frazzle from doing 2 shifts in O/R, Hawkeye and Trapper hatch a scheme to prevent Frank and Hot Lips from leaving. That night, Hawkeye and Trapper pretend they have found gold, letting Frank overhear them. Frank then withdraws his request when he thinks he's found gold himself, although the joke is on him when he finds, amongst other things, a gilded jeep!

General Clayton calls to say that a ceasefire is to be declared. The camp celebrates, Klinger gives away his dresses and locals start to take pieces of the camp. But Trapper does not believe it. Hawkeye claims he is married to avoid promises he made to several nurses. The party to celebrate the cease-fire, which never really took place, is interrupted by incoming wounded.

Captain Kaplan is to be shipped home, but becomes paranoid that something will happen to him before he leaves. He takes the wheel of the jeep to drive to Kimpo himself, but crashes and ends up in plaster. Henry's wife is in labor and gives birth while he has Radar calling the hospital every 5 mins.An entertainer, Jackie Flash, visits the camp to entertain the troops.