After a landslide in the Axenstrasse, geologist Mara Graf and her boyfriend, seismologist Gian Wyss, are investigating for the Seismological Service of the University of Technology in Switzerland. For Gian this is a good opportunity to spend some weeks of Spring at home. For Mara to be with her grandmother and sister. The next day Mara Gian want to anchor indicators at the bottom of Lake Lucerne in a dive. They decide to dive alone. Before Mara could follow him, an explosion rocked their ship.