Emily and Alexander travel to Germany to visit their cousin Fritz in Friedrichshafen on Lake Constance. Fritz is the mouse of Count Zeppelin's granddaughter Leni, who is just presenting his latest invention to the public: an airship called a Zeppelin. However, his 1st officer Zsolt is an agent of a foreign power. He is supposed to get hold of the plans of the airship and destroy the zeppelins that have already been built. However, he has not reckoned with Leni and the three mice.