In the 25th century, under the leadership of the League of Peoples, war and crime are a thing of the past and life is held sacred. That is, as long as you're healthy and beautiful. But those who are deformed, flawed or misfit in any way are destined - or is "doomed" a better word? - to become Explorers, crews assigned to probe worlds so hostile, the chances of returning are somewhere between slim and none.
A qualified member of the expendable Explorer Corps due to her untreated facial blemish, Youn Suu sets out on a standard suicidal mission. Along with her partner, Tut, Youn is tasked with investigating a sudden infestation of the Balrog--a sentient red moss that can form parasitic, symbiotic relationship with its host--on the home world of the Cashlings.
The mission takes a turn for the worse when Suu is infected with the Balrog. But just before all is lost, Suu and Tut are rescued from the planet by legendary Expendable Admiral Festina Ramos. Aboard an Outward Fleet starship, they find that the Balrog is far more intelligent and sinister than they ever could have imagined. It is only then that the scope and danger of this nightmare is truly revealed.