Hayakawa Yui is totally unmotivated, lazy girl who loves sleeping and eating. She's 16 but looks like a boy from elementary school; her one and only talent is to run really fast.

After accidentally switching on the time machine which her younger brother made she travels to the Sengoku period, where luckily joins to retreating foot soldiers using the name Yuinosuke. Then she eats some amanitas and meets the man of her dream.