Felix and the river man

12-year-old Felix who, on a field trip to the National Museum with his 6th grade, ends up together with his classmate Siff down in the museum's basement, where they find a secret room filled with all kinds of things from the mythical creatures that people in Denmark once believed in. When Felix bumps into a bookcase, a small flute falls into his bag without him seeing it. When Felix comes home, he discovers the flute and finds out that he can hypnotize people by playing it - and at the same time awakens the river man from 143 years of slumber.