On a hot July morning on Sweden's idyllic vacation island of Sandhamn, a man takes his dog for a walk and makes a gruesome discovery: a body, tangled in fishing net, has washed ashore.

Police detective Thomas Andreasson is the first to arrive on the scene. Before long, he has identified the deceased as Krister Berggren, a bachelor from the mainland who has been missing for months. All signs point to an accident--until another brutalized corpse is found at the local bed-and-breakfast. But this time it is Berggren's cousin, whom Thomas interviewed in Stockholm just days...