12-year-old Karli discovers an incredible secret in his parents' hotel: a hidden floor that takes him straight back to 1938. There he meets Hannah, a Jewish girl, and Georg, a young shoe shiner with a big heart. The three become friends. But soon they find themselves in the middle of a mysterious criminal case. In the present, Karli's parents become skeptical because Karli spends so much time hunting criminals. Slowly, Karli realizes what danger threatens his two friends in the past.

If 12-year-old Karl had been able to choose who he fell in love with, it probably wouldn't have been Lea, who is significantly taller than him. The fact that Lea is about to change schools doesn't make things any easier. "Different school, different boys!" says Karl's older brother Tom, advising him to be brave. Karl invites Lea to his improvised bathroom laboratory. Photography and developing pictures has been Karl's favorite pastime for years. Lea is impressed by Karl's "light magic" in the lab, but time is running out for further meetings. All that remains is the project trip, during which Karl's class has decided to make a workshop film with the working title "In Love." Suddenly, it becomes apparent that Karl is not alone with his problem.