The U.S.S. Enterprise goes to the planet Dramia to deliver medical supplies, only to have McCoy arrested and put on trial for deliberately spreading a plague that killed most of the population of the colony Dramia II. Nineteen years earlier, Dr. McCoy headed a mass-inoculation program there against Saurian virus. This disaster occurred after his team left the planet. Now Kirk must clear his friend's name before the doctor is executed.