Jang Dan-bi is an indecisive high school senior who has given up on math, like many others in Korea, and is just drifting through life without dreams or passion. On the day of the college entrance exam, which is the most important day of her life, Dan-bi, plagued by anxiety, runs away from the exam hall under the pressure. She wishes on the rain for just one chance to disappear, and the rain grants her wish! Dan-bi ends up falling into Joseon through a puddle. Meanwhile, in Joseon, a ritual is being held to pray for rain amidst a severe drought that has lasted for three years. Since the young King Yi Do ascended the throne, the drought has continued relentlessly, putting his reign in a precarious situation! During the rain ritual, Dan-bi, drenched and appearing out of nowhere, shows up like a goddess. Yi Do begins to learn mathematics from Dan-bi, who came from the future, and their secret lessons begin until the next rain falls!