In a remote karst forest along the China–Vietnam border, fewer than 80 Cao-Vit Gibbons survive, one of the world’s rarest primates. They leap, call, and interact in secretive ways, while their habitat faces constant pressure from goat grazing and human activities. Over more than a decade, Chinese and Vietnamese scientists and researchers have studied these gibbons and implemented cross-border conservation efforts. Filmed over several years, this documentary captures the gibbons’ hidden lives and tells a story of survival, resilience, and international collaboration to protect one of Asia’s most extraordinary primates.