A young Army nurse, Lieutenant Ruth McCara (June Allyson), is newly assigned to the 8666th MASH, a mobile field hospital constantly on the move during the Korean War. Ruth's personal mission is to serve suffering humanity. She initially experiences an uncomfortable welcoming by the unit's hard-drinking, no-nonsense chief surgeon, Major Jed Webbe (Humphrey Bogart). Jed engages in a helicopter rescue of army casualties while under fire. He is a much-tried doctor by the continual movement of the outfit due to the changing battle lines. Responsible for the dismantling and re-pitching of the tent hospital is Sergeant Orvil Statt (Keenan Wynn), a former circus roustabout.