In Missing in the Mekong, the nineteenth in the Wallace Boys series, Nigel and Bruce, together with Singaporeans Kheng Peng and Zainal, while returning to Singapore from the Sulu Sea aboard the Silver Spray, encounter a small boat bobbing about on the wide expanse of the South China Sea, apparently abandoned.This chance encounter takes the four boys to the shores of Vietnam, where they help to find an American soldier, Marvin Wynne, left over from the Vietnam War under tragic circumstances.