From Etc:

Ryo thinks she is a normal girl living a normal high school life in modern day Tokyo. But is she? On a school trip, a mysterious man attacks her and claims that he is Benkei, the legendary warrior monk who collected 999 of his opponents' swords in battle during the Heinan era. And he also claims that she is Ushiwakamaru, the 1000th opponent, that according to folklore, beat Benkei and became his master. But...Ushiwakamaru was a man! And Benkei and Ushiwakamaru existed in 13th century feudal Japan!