Joan, Duchess of Hohenstein, was arranged from birth to marry the Prince of Courtland. She secretly visits Courtland dressed as a man to inspect her fiance and finds him much to her liking, and Princess Margret thinks about as highly of “Count von Loen”, bitterly angering Muscovite Prince Wasp, who has claimed Margret as his own. On the wedding day, Joan discovers that the man she had taken for the prince was actually his younger brother. The actual Prince of Courtland is a sniveling old man, commonly called about town Louis the Craven for what the townsfolk see as selling out their Germanic freedoms for the autocratic protection of Russia. Joan flees back to Hohenstein and a Russian-backed battle ensues to capture her.
Their supplies running low, a conspiracy is hatched by the high ministers in Hohenstein. Joan is abducted and taken to a place of safety on the Baltic coast. Meanwhile, Maurice von Lynar, a Danish soldier in Joan’s army who bears a remarkable resemblance to her, puts on her dress and feigns capitulating to the invaders so that they’ll withdraw and Hohenstein can be reinforced. The false Joan is taken to Margret, who she takes for Count von Loen. They are married that day by the castle priest. Discovered, Maurice is sentenced to be torn apart by four wild horses.
Joan, trapped on Isle Rugen, finds that her hostess is Theresa von Lynar — Maurice’s mother. She also finds that Maurice is, in fact, her half-brother, her father having secretly married Theresa after his first wife’s death. At the same time, Conrad — the younger Courtland prince Joan thought she was betrothed to — shipwrecks on the island. He and Joan fall in love, though he is a priest and Joan is already married. “In name only”, Theresa says to both.
Word reaches them of what’s happened in Courtland. Joan, Theresa, Conrad, and the handful of guards they have hasten there. The people of Courtland, who find the spectacle playing out before them abhorrent and un-Christian, hail Conrad as a liberator and the true prince. The Courtland military turn and the Russians retreat from them and the well-armed mob. They rally and intend to invade Courtland with the whole of the Czar’s army and annex it into the empire. Courtland holds off the invasion as long as they can, waiting for reinforcement from Plassenburg. Theresa buys them much-needed time by suicide bombing the Russian encampment, killing both Prince Wasp and Louis.
In the end, the Russians are repelled. Conrad, released from his vows as the new reigning prince of Courtland, marries the widowed Joan. On Princess Joan’s abdication as duchess, Maurice and Margret becoming the new duke and duchess of Hohenstein.