At his uncle’s behest, the young Karl-Heinrich, Hereditary Prince of Saxe-Karlsburg, is sent to Heidelberg to study for a year. After a joyless youth in the gloomy, stately Karlsburg Castle, the prince embarks on the most carefree period of his life. He falls hopelessly in love with Käthi, the charming daughter of his landlords. But duty calls: the prince is gravely ill, and Karl-Heinrich must return to assume his duties as heir apparent and future ruler.