Yoko's family lives in northeastern Korea, though they are Japanese, because her father is a Japanese government official in Manchuria during World War II. Yoko and her family live 50 miles from the Manchurian border, so her father can come home to visit them occasionally. Manchuria was the site of fierce fighting during the war, so the family lives in Korea, which has been more peaceful to date, instead.
The book opens in July of 1945. As the war goes on, Americans begin to bomb Korea, and Yoko sees their B-29s above. In addition, Koreans, who are opposed to the Japanese presence in their country, form the Anti-Japanese Communist Army. As the Soviet army threatens to invade Korea, Japanese families are forced to board a train to leave Korea to return to safety in Japan. Even though the Soviets did not officially declare war on Japan until August of 1945, after the bombings of Hiroshima and Nagasaki, people believed that the Soviets were going to attack Korea in July of 1945.
Yoko's family lives in Korea because Japan has conquered and occupied Korea and a part of northeastern China called Manchuria. Japanese people are ruling the country now. Yoko's father is a doctor -- some say he is involved in some horrific experiments on human beings in Manchuria, but that is not known sure.
As the war goes on and Japan starts to lose, being Japanese stops being a good thing for them and begins to be a danger. The Koreans hate the Japanese colonizers because of the way they have treated the Koreans and they try to take revenge on them as they leave, for example.