"I don't pamper my cousin, nor do I even like her."

Duke Einkline's son Hartmut has a younger fiancée with whom he's been in a relationship for nine years. They love each other and plan to register their marriage once she graduates from high school, but there's one troublesome problem: his paternal cousin keeps pursuing him and repeatedly interfering with his meetings with his fiancée. Then, finally attending his fiancée's graduation and accompanying her to the prom that followed, Hartmut is inexplicably summoned to the stage by the Third Princess, who is supposed to declare the opening of the prom. "I'm bringing charges against you!" "Huh?"