Bo and Kristin Jonsson embraced their time as expats in China. Then their son ended up in a Beijing jail.

The Swedish family had moved to Beijing in 2007 for Mr. Jonsson’s job at Ericsson. They lived in a suburban compound near the international school their four kids attended and were part of a vigorous social life with other expats. “It’s a bubble,” said Mrs. Jonsson. “Yet, we lived a very ordinary life.”

By the fall of 2013, the Jonssons’ daughter was still in middle school in Beijing but the two older sons had returned to Sweden to start university and the Jonssons were starting to consider moving back, too.

Their youngest son, then-18-year-old Noak, had graduated from high school—he was in the same year as my son and the two have many friends in common—and was spending a semester studying Mandarin at Peking University. He was planning to start university in Sweden the following fall.

A Wednesday evening in November changed all his plans.