The Amur River is almost unknown. Yet it is the tenth longest river in the world, rising in the Mongolian mountains and flowing through Siberia to the Pacific. For 1,100 miles it forms the tense border between Russia and China. Haunted by the memory of land-grabs and unequal treaties, this is the most densely fortified frontier on earth. In his 80th year, Colin Thubron takes a dramatic journey from the Amur's secret source to its giant mouth, covering almost 3,000 miles. Harassed by injury and by arrest from the local police, he makes his way along both the Russian and Chinese shores, starting out by Mongolian horse, then hitchhiking, sailing on poacher's sloops or travelling the Trans-Siberian Express. Having revived his Russian and Mandarin, he talks to everyone he meets, from Chinese traders to Russian fishermen, from monks to indigenous peoples.