The series was designed according to the following premise: Students like the feeling of accomplishment in having read an entire book in the foreign language - even beginners who usually have to wait until they become more proficient.
The main character is consistent: Private eye Helmut Müller and his secretary Bea Braun encounter various mysteries and crimes in Germany, Switzerland and Austria. Each time, they are called upon to solve the puzzle as the reader follows their adventure. The books lend themselves to different students reading various books and reporting to one another. There is an exercise section in the appendix and cultural notes to help the reader understand the content. The series has three different levels of difficulty.