Blaine and Tracey, the Sassafras twins, were to spend the summer at Camp Zip-Fire. However when they failed their science class, their father sent them to spend the summer with Uncle Cecil, a well-known research scientist. Their uncle and his assistant President Lincoln, a prairie dog, plan for them an invisible zip line journey around the globe where they encounter and learn about amazing animals in such places as the African grasslands, the Amazon rainforest, and many other destinations.