The book takes place shortly before the beginning of WW2. It is about a teacher, who doesn't think the way he is supposed to think during that time, but acts as if he does to avoid loosing his job. Not long after the beginning of the book he goes to a kind of camp with his students (where they learn to march and shoot, like a training camp for young soldiers).
Anyway, in the second or third night of camp a camera from one of the pupils is stolen. The teacher decides to watch camp the next night and sees how on of his students receives a letter. In order to find out about the letter he reads the students diary and gets to know that his student is having an affair with an outlaw girl from the woods. He is planning on teaching the student a lesson, but all his plans break when one of his pupils is murdered.
The outlaw girl is accused of the murder, but for the teacher it is clear, that she is innocent. So he starts looking for the real murderer himself.