Sherlock Holmes receives an invitation from his brother Mycroft to attend the premiere of an operetta in Berlin together. However, the brother of the world's greatest detective is actually on a secret mission as a high-ranking official of the British Foreign Office. He is tasked with trying to prevent the conclusion of a highly dangerous naval pact between the German Empire, Austria-Hungary, and Italy. While Mycroft dedicates himself to his delicate mission in the German capital, Sherlock Holmes, accompanied by his travel companion Dr. Watson—who could only be lured by the promise that this trip is not about solving a criminal case—attends the performance of the operetta "Wie einst im Mai." They join an illustrious circle around the composer Walter Kollo and meet Leo Fall, Jean Gilbert, and Paul Lincke.