 A Viennese opera house, early in the century. In attendance are lieutenants Kaiser and Lobheimer. Two young ladies on the balcony, Mizzi and Christine, drop their opera glasses, hitting one of the officers. The Baron von Eggersdorff arrives in his box. Lobheimer leaves early for his tryst with the Baron s wife. The Baron soon arrives home, in a suspicious mood. Lobheimer rejoins Kaiser in a café with the two girls. Lobheimer soon falls for Christine... The Baroness wonders why her lover has been absent for so long; the two now part for good. But the Baron learns the secret and discovers that a key still in his wife s possession opens the lieutenant s door. He challenges Lobheimer to a duel...