Excursion to Zagreb with the quirky geography teacher McFinnegan – what initially seems like just another ordinary day at the flying school soon turns into the next big adventure for the ACE Scouts. The four of them set out to track down a mysterious phantom said to dwell in the catacombs beneath the Zagreb theater. As they enter the underworld through a mirror, they encounter not only a legendary device of the brilliant inventor Nikola Tesla but also a hundred-year-old monster.