On a pleasant New Hampshire summer day in 1950, an inexperienced US Army Reserve Surgical Nurse nestles at home with a newborn infant recently abandoned by her alcoholic sister-in-law. Just weeks earlier, North Korean troops streamed across the 38th Parallel of the Korean Peninsula, sending the South Korean and American Armies reeling. Unable to receive an exemption because of President Truman's Declaration of National Emergency, nine days later Lieutenant Mary Belanger leaves the baby she came to love and reports for duty with the 100th MASH (Mobile Army Surgical Hospital) close behind the enemy lines.

Shocked by the brutality of endless combat wounds, Mary Belanger gets "wet" on her first day of surgery, forcing her to grow capabilities she never thought possible. Along with unending surgeries, she experiences emergency bugouts and is part of an amphibious invasion that turns the war around. Her medical capabilities grow while becoming a leader among the nurses. A sexual assault by one of the doctors creates new trauma, particularly when the senior nurse dismisses the event as boys will be boys. News from home brings more news about her philandering husband.

When the war looks won, Chinese troops intervene and throw Allied Units back. Mary is trapped with her surgical partner and another nurse behind enemy lines. Although seriously injured, she manages to escape and is evacuated to Japan to recover. Once well enough, Mary uses her recovery time to make a surprise visit home to resolve domestic issues before returning to her unit as the newly appointed Senior Nurse.