In 1946, the British biochemist Joseph Needham returned from a four-year stay in China. Needham scholars have considered this visit as a revelatory period that paved the way for his famous book series Science and Civilization in China (SCC). Surprisingly, however, Needham's actual time in China has remained largely unstudied over the last seventy years. As director of the Sino-British Scientific Cooperation Office, Needham travelled throughout Free China to promote cooperation between British and Chinese scientists to contain the Japanese invasion during the Second World War. By rediscovering Needham's peregrinations, this paper re-examines the origins of his fascination for China. First, it contests the widely held idea that this Chinese episode is quite separate and different from Needham's first half-life as a leftist scientist. Second, it demonstrates how the political and philosophical commitments he inherited from the social relations of science movement, and his biochemical research, shaped his interest in China's past. Finally, this paper recounts these forgotten years to reveal their implications for his later pursuits as historian of science and as director of the natural-science division of UNESCO. It highlights how, while in China, Needham co-constituted the philosophical tenets of his scientific programme at UNESCO and the conceptual foundations of his SCC.