Makoto Niwa has to live with his aunt while his parents are overseas, which he views as the perfect opportunity to live the life of the typical Japanese teenage male. He meticulously keeps tabs on his experiences as either negative or positive as a way of grading his progression. However, his plan for a perfectly normal adolescence did not take Erio Touwa into account. Upon moving in to his aunt's house, he discovers a cousin he never knew he had. She's the same age as Makoto, and really weird, to boot. Erio is convinced that she is an alien and her mission is to study humanity. She tends to stay wrapped up in a futon, and walks around town barefoot. Needless to say, this strange addition to his family is a stumbling block for Makoto’s perfect life. Rather than his ideal adolescence, Makoto will instead live a more interesting life than he ever could have imagined alongside Erio in Denpa Onna to Seishun Otoko.