Der Wannsee-Mörder (2002)

Johanna, a beautiful brunette, comes back to her father's house after accomplishing a training course in a car factory in Turin. At home, she reads a note her father had ensconced in the welcome bouquet of flowers: «You're back at last.» Then, she meets her best friend, Jasmin, and they go together to see their group of friends # all have grown up together by the lake. It comes up they have all received a message similar to her own. Next evening, these young persons are having a good time, dancing, flirting, drinking a lot, as a retarded person watches them, and evokes a memory of a similar evening years ago... Then, people start dying around, and in the peaceful lake that hides mysteries they share.

Unfortunately the movie Der Wannsee-Mörder is not yet available on HBO Max.