Set during the 1920s, Haikara-san ga Tooru follows the spirited and athletic tomboy Benio and her dynamic with the dashing half-German soldier, Shinobu. Initially at odds, Benio gradually warms up to Shinobu after he comes to her aid during a series of mishaps. To her astonishment, Benio learns of her arranged marriage to none other than Shinobu himself. Their journey unfolds with challenges as Shinobu is called to battle in the war, Benio grapples with the idea of matrimony, and navigates relationships with her future family and employer. Amidst these trials, the looming threat of the catastrophic Kanto earthquake adds further complexity to their intertwined fates.