German schoolboy Paul Haller is bullied since his dear, unconventional father Thomas disappeared, people doubting if the unpopular 'crank' really was on an expedition to find the legendary cave in Ursulenberg mountain. He strikes a friendship with corpulent, moody halfwit Max 'Fatboy'. Having studied and brought Thomas's notebook, Paul takes buddy Max along, escaping from the hike on the last school day, to prove and complete father's expedition. The perilous journey tests their unequal friendship and courage, while the mayor orders a search party which they carefully avoid. Reaching the cave is the start of a last, new set of challenges for all.
—KGF Vissers