Nagami Yuu finds out that his little sister Nagami Suzuka - who is her highschool student council president - wrote a light novel titled "The Story of the Little Sister Who Loves Her Older Brother so Much it's Troubling" on a whim under the pen name Towano Chikai (meaning "Eternal Vow"????, and won an award.

He is a bit taken aback at first, because he had been submitting light novels to contests for years without winning an award, but agrees to serve as her stand-in by posing as Towano Chikai, so that their father and her school won't find out about her new part-time job.

But it's not like she likes her brother, or anything!