Explorers in a hot-air balloon land on an island, figuring that they must be the only inhabitants. However, they discover a bullet inside a wounded animal--one which must have been fired within the previous three months. The men propose to build a canoe so they can survey the island in search of other human life. Many adventures follow, one after another. They find a large chest filled with provisions and tools. Setting off in search of who might have left the chest, the travelers make their way through the Mercy River to the sea. During their trek, the men find remnants of the balloon they arrived in. Back at camp (Granite House) they find their ladder to the house has been removed--by invading orangutans. Soon the animals are defeated, except for one, whom they tame to become a house servant. The men construct a bridge over the river. They protect their abode by surrounding it on all sides with water. They undertake projects to make their colony habitable and comfortable. They create a hydraulic lift to replace the ladder. They build a seagoing boat for further exploration. Eventually they discover another human on Tabor Island, bringing him back to their now-well-stocked colony.