The time is Sengoku Ransei (Warring States Period). As Oda Nobuyami attempts to unify the country, Muramasa, a demon sword, aims to be the best in the world. The sword is a sword with a will, but it possesses a strong mystical power that causes its wielder to lose his or her senses. When at last a wielder who can withstand Muramasa's demonic power appears on the scene... it is a baby, Nobumitsu! Nobumitsu, who was abandoned by his father Nobuyaku because of his kindness and weakness, wants to become strong and be recognized by his father, so he and Muramasa resonate, and the strongest of the two are born!　But what happens when Nobumitsu and Muramasa are pursued by Nobumitsu and Muramasa?　This is a buddy-battle action of "Youtou x BABY"!