In 1958, a diary is found in a cave on the small Japanese island of Hatoma. Alongside it are the remains of three people.

The journal reveals the story of Hiromi, a sixteen-year-old girl who’d grown up in the United States before living in Japan in the midst of World War II. One day, while collecting star sand—tiny star-shaped fossils—Hiromi finds two army deserters hiding in the seaside cavern—one American, one Japanese. The soldiers don’t speak the same language, but they’ve reached an agreement based on a shared hope: to cause no more harm and survive. Hiromi resolves to care for the men—feeding them and nursing their ailments—despite the risk that, if caught, she’ll die alongside them as a traitor. But when a fourth person joins in on their secret, they must face a threat from within. The diary abruptly ends, leaving everyone’s fate a mystery.

Decades later, in 2011, a young female university student decides to finally determine who died in that cave and who lived. Her search will lead her to the lone survivor—and bring closure to a gripping tale of heroism at a time when committing to peace was the most dangerous act of all.