The story takes place in Nanyou Campus where the violent high school heroine, Sonsaku Hakufu has transferred to. She possesses the spirit of the Koutou Champion, Sonsaku who passed away at the age 26. The Sonsaku who is destined to fight. Shuuyu Koukin is worried about Sonsaku so he watches over his childhood friend. But, because of him worrying, Sonsaku just gets excited about the battles.

However, an order was issued to defeat Sonsaku and so she is attacked by strong enemies one after the other. Sonsaku was able to get through all the battles due to her strength but, before long, she fights with one of the top 4 fighters of Nanyou, Ryomou Shimei. But, in the dark lies a conspiracy for hegemony. What will become of Sonsaku Hakufu's fate....!?

"Ikkitousen" refers to strength of how one person can fight a thousand people at once.