Zhang Qian
Captured for ten years on a diplomatic mission, he came home having opened the Silk Road
Zhang Qian was a palace official when the Han emperor Wu sent him west around 138 BC to find the Yuezhi people and propose an alliance against the Xiongnu, a nomadic empire that had been raiding China's northern border for generations. Almost as soon as he left imperial territory, Xiongnu horsemen captured him and his party. He was held for about ten years, given a Xiongnu wife, and by most accounts treated as a hostage rather than a prisoner, but he never abandoned his mission.
He eventually escaped westward, still carrying the emperor's token of authority, and reached the Yuezhi in what is now Central Asia, only to find they had settled comfortably and had no interest in fighting the Xiongnu. Captured again on his way home and held for roughly a year, he finally reached the Han court around 125 BC with something more valuable than an alliance: detailed first-hand knowledge of kingdoms, markets and roads across Central Asia that China had only heard of in rumour.
His report described Ferghana's prized horses, the wealth of Bactria and routes toward India and Persia. It directly shaped Han policy toward the region and is credited with opening the network of trade routes later known as the Silk Road, connecting China to Central Asia and, eventually, the Mediterranean world.