Rob is on his way home from vacation when his plane goes down over the ocean biome. He eventually reaches shore, but where is he? And what new dangers lie ahead? His attempts at survival barely keep him alive until he allies with Frida, the sole inhabitant of a lonely stretch of beach. Rob learns enough from Frida to survive, but he misses life on his family's horse ranch. Determined to find his way home, he sets off to cross the Extreme Hills--and steps into the middle of an ongoing war. The evil Dr. Dirt has enchanted battalions of skeletons to invade every biome and hold every boundary in an attempt to rule the Overworld. Now peace-loving Rob and stealthy Frida must put together their own army and save humanity if Rob is ever to see his home and family again. Using his skills in taming and training horses, Rob forms a cavalry and recruits the friends he makes along the way. Turner and Stormie are naturally brave and quick-witted. Jools and Kim need encouragement but have a way with animals and can unravel even the toughest problems. Unfortunately, Dr. Dirt has a surprise for the ragtag unit; he's mounted his skeleton soldiers on zombie horses, and the combination spells terror for Rob and company.