The cast of characters in M*A*S*H were all members of the 4077th Mobile Army Surgical Hospital, stationed behind the lines during the Korean War. Their job was to treat the wounded being sent to them from the front lines. The environment was depressing; many of the doctors could not really believe that they were living under the conditions to which they were being subjected. There was an overwhelming sense of futility and insanity of war that permeated their daily lives. A certain sense of humor was necessary for survival.

Most of the senior members of the M.A.S.H. unit had wives and families back home, but that never stopped them from propositioning every good-looking nurse that they could con into their quarters. Two of the surgeons were Hawkeye Pierce and Trapper John McIntyre. Like virtually everyone else, they were always breaking regulations. Hawkeye, despite his escapades, was probably the most intellectual of the doctors and was sometimes seen musing on the dehumanizing nature of war and questioning its moral validity.

Among others who were featured was Frank Burns, who was possibly the worst doctor in the unit, and the constant butt of practical jokes perpetrated by Hawkeye and Trapper because of his arrogance and his feigned adherance to military regulations. Hot Lips Houlihan was the head nurse who, despite her admonitions to both her nurses and doctors about fooling around with each other, had been having an affair with Frank Burns for an extended period. Henry Blake, the commanding officer whose prime concern was the work of the doctors in the operating room, couldn't care less about what they did during their free time. Radar O'Reilly was the extremely shy and bumbling young aid to Colonel Blake.

There were changes in cast over the years. The first significant addition was that of Cpl. Maxwell Klinger, an aide to the doctors in the operating room. There was nothing really wrong with him: it was just that he always dressed in women's clothing in a desparate, though futile, attempt to get himself discharged as mentally unfit. In 1975 Col. Blake was replaced by Col. Potter, who was somewhat more sardonic and definitely less silly than his predecessor. In 1976 B.J. Hunnicut replaced Trapper as Hawkeye's tentmate and co-conspirator. In 1977 Major Burns was replaced by an aristocratic Bostonian, Major Charles Emerson Winchester.