The USS Enterprise-D has been on its mission of exploration under the command of Captain Jean-Luc Picard for less than a year when a virulant epidemic strikes Archaria III, endangering thousands of lives and provoking mob violence as panicking residents retaliate against those they believe responsible for the spread of the new disease. Captain Picard and his crew are dispatched to the troubled planet to deliver humanitarian aid. While Data and Natasha Yar team up to uncover the true origins of the virus, Dr Beverley Crusher finds that the rapidly spreading sickness resists all her efforts to come up with a cure. Then their desperate quest becomes even more of a race against time when crewmember Deanna Troi succumbs to the dreaded plague . . .