How far can you run away from the inevitable? Dan Bi is in high school, hates math and has already buried her future dreams because they require math! On the day of her university entrance exam, she is so confused that she runs away. On this particularly rainy day, she jumps into a puddle - and suddenly finds herself traveling back in time to the Joseon Dynasty, where she meets the young ruler Lee Do. When she tries to explain to the royal court that she is a “gosam” (high school student), she is mistaken for a eunuch who is capable of finally bringing the necessary rain to a country plagued by persistent drought. Can Dan Bi save her life by being of use to the king - or will she have to use math to do the job?