Kid detectives Lasse and Maja put their crime-solving skills to the test for the first time when a valuable, ancient text is stolen.

In the 11th century a missionary goes missing somewhere in the huge forests bordering the northern parts of Sweden. Among the rescue party chosen to find him is Nanna, a young woman on her first real mission and her first return to the part of the country where she was born. But what they will find deep inside those woods is something else entirely. Something dark. Something ancient. Something evil.