It is October 1914, and Swedish naval officer Lars Tobiasson-Svartman is charged with a secret mission to take depth readings around the Stockholm archipelago. In the course of his work, he lands on the rocky isle of Halsskär. It seems impossible for it to be habitable, yet it is home to the young widow Sara Fredrika, who lives in near-total isolation and is unaware that the world is at war. A man of control and precision, Tobiasson-Svartman is overwhelmed...