Nagami Yuu finds out that his little sister Nagami Suzuka who is a high school student council president wrote a light novel titled "The Story of the Little Sister Who Loves Her Older Brother so Much It's Troubling" under the pen name Towano Chikai (Eternal Vow) and won an award. He is 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.