A young boy named Shinichi, who is frequently bullied, flees his problems and prays for courage at a great oak tree. His wish unexpectedly sends him back in time to the Sengoku period of 1574, where he materializes in the midst of a violent battle. He inadvertently saves the life of Ijiri Matabe, a feared samurai general known as the Demon Ijiri, who leads the army of the small domain of Kasuga.

Welcomed into Kasuga, Shinichi is astonished to meet Princess Ren, the lord's daughter, who is the spitting image of a woman he has seen repeatedly in his dreams. Princess Ren and Matabe are childhood friends who share a deep, unspoken love for one another, but their difference in social status prevents them from ever acting on their feelings. Their quiet longing is thrown into crisis when a powerful and hot-tempered neighboring lord, Okurai Takatora, becomes famous for Ren's beauty and demands her hand in marriage. To protect her people, Ren feels she cannot refuse, and the prospect of losing her forces Matabe to watch helplessly as his beloved is promised to another.

Meanwhile, Shinichi's frantic parents in the present day trace his disappearance and manage to travel back in time themselves, arriving in Kasuga in their sport utility vehicle. They bring with them astonishing news from the future: historical records show no mention of the Kasuga domain, suggesting that all their battles and political struggles may be ultimately fruitless. Armed with this revelation, Princess Ren is released from her duty to marry for political security, choosing instead to follow her heart and declare her love for Matabe. Her refusal enrages Lord Takatora, who is unaccustomed to rejection, and he launches a massive army to destroy Kasuga and claim Ren by force. As the two domains clash in a decisive and grand-scale battle, Shinichi and his family must use their wits and modern knowledge to help Matabe and Ren fight for their future and their forbidden love.