In 1986, fourteen-year-old twins Samson and Jonah travel from the Sunshine Coast to the wild backcountry of Tasmania to live on a mountain with a granddad they've never met. Clancy Fox is a beat-up old man obsessed with finding his long-missing daughter, River. He's convinced that she was taken by a Tasmanian tiger pack. The resentful, brooding Jonah and thoughtful, inquisitive Samson become entranced, in different ways, with the mountain. While Samson - who has Down syndrome - finds mystery and delight all around, Jonah develops a dark obsession as persistent as Clancy's desire to bring River home.