The Emperor of Japan, suspecting an ulterior motive in the American battleship fleet's world voyage, sends his cousin, Prince Maiyo, on a mission to find the secret, if one exists. The Prince arrives in London, is hosted in high society, and attracts the interest of several women, most notably Miss Penelope Morse, an American by origin but has spent the majority of her life in England. Iwo murders occur, with both victims being Americans serving in the diplomatic service. The primary suspect is Prince Maiyo. Is he really guilty?