John Carter, mighty Warlord of Mars, is taken to the ill-omened city of Morbus. There he meets Ras Thavas, evil genius and master surgeon. A man who has succeeded in his nightmare wish of creating life in his own beings - creatures that ultimately rebel and threaten the lives of Ras Thavas, of John Carter and all of Mars.

From Phundahl at their western extremity, east to Toonol, the Great Toonolian Marshes stretch across the dying planet for eighteen hundred earth miles like some unclean, venomous, Gargantuan reptile-an oozy marshland through which wind narrow watercourses connecting occasional bodies of open water, little lakes, the largest of which covers but a few acres. This monotony of marsh and jungle and water is occasionally broken by rocky islands, themselves usually clothed in jungle verdure, the skeletal remains of an ancient mountain range.