A fisherman is burned alive when his boat suddenly explodes in Lake Constance. In the wreckage, the police find the eerie mask of a Celtic deity and a cryptic message in a strange, elusive language.
Because of these mysterious leads, chief inspector Michael Oberländer from Lindau (Matthias Koeberlin) and inspector Hannah Zeiler from Bregenz (Nora von Waldstätten) cross paths – two detectives who could not be any more different, but in this trans-border case, they are forced to come to an arrangement.
Their investigation leads them to the deceased’s brother, Ludwig Pfeilschifter (Stephan Kampwirth). Fifteen years ago, he and his wife Christa (Doris Schretzmayer) were the ones to find three Celtic masks in a nearby mountain crevice.
When shortly thereafter Christa’s father Walter (Karl Fischer) is found slashed and hung to a tree with the second mask, they need to prevent the murder with the third mask and the by now deciphered message “I have killed”.