Margaret Sandahl believes she knows the challenges she faces when she leaves Earth on the first manned expedition to Mars. But she takes solace in her assumption that the Earth she leaves behind will be there, unchanged, when she returns three years later. A deadly virus changes everything. While the Arean Explorer speeds through space to its rendezvous with the red planet, researchers at the International Center for Disease Control detect a new encephalopathy, a lethal brain disease whose earliest symptom is loss of judgment. On Earth, ICDC directors Angie Munro and Mike Novak try to control a burgeoning epidemic, while Margaret and her twelve crew mates, face dangers they could never have imagined when one of the crew brings the virus aboard their craft, bound irreversibly toward the plains of Mars.