Edo Nanami is a university student who is knowledgeable about history thanks to her beloved grandfather. One day, rookie detective Oshima Haruto, whom she secretly has a crush on, asks her to help investigate a series of incidents in which people have been hit with water balloons. The victims' names are "Oishi", "Fuwa", "Hazama", "Kataoka", and "Yoshida", and they all have nothing in common apart from having been hit with water balloons. How will Nanami track down the culprit with only limited information? And what will be the unexpected outcome of the incident? (Source: Japanese = fujitv.co.jp || Translation = MyDramaList)