After her mother remarried, high schooler Honoka, who's a bit of a delinquent, gained a step sister older than her - Nagisa.

Having a new family so suddenly made Honoka rather uneasy, and she sought to put some distance between herself and Nagisa. However, in the face of Nagisa's kind and welcoming nature, she gradually opens her heart up to her.
On the other hand, however, she's rather annoyed by Nagisa fussing over her all the time.

One day, while she's out with her friends like usual, Honoka stays out late playing and ignoring Nagisa's warning. As a result, she's approached by some drunk college students.
Even after being saved from a dangerous situation by Nagisa, Honoka shows no signs of remorse. Seeing her attitude, Nagisa finally gets angry.
"I'll smack your butt! Okay?!"

Faced with the furious expression of the usually gentle Nagisa, and the strict punishment of 100 spanks, Honoka comes to strongly regret her actions.
"This person is scary when she's angry..."

Cover + 29 Pages of main story content