Danbi is a senior high school student taking the College Scholastic Ability Test (CSAT). However, she is not good at mathematics and is reluctant to take the exam. On exam day, she decides to run away and stops by a playground, where she hears a drum from a puddle of water. As she has the ability to travel through spacetime, she takes a leap of faith and jumps into the puddle of water and into the Joseon era. In Joseon, she is mistaken for a eunuch and becomes a confidante of King Lee Do. Danbi finds that in the Joseon Dynasty, her high school level of math and science makes her the greatest scientist in the entire Kingdom. She falls in love with the King and inspires him in his goal to reform the country. However, fearing for their own privileges, Danbi becomes the target of traditionalist courtiers who seek her death. Having a hand in saving both the King and the Queen, Danbi becomes friends with the Queen who, in order to allow the King and Danbi to be together, endorses Danbi becoming a royal consort. Presented to the Queen Dowager, who is a doppelganger of her own mother, Danbi becomes very home sick and therefore decides to return to her era. Lee accepts her decision, and they leave, heading for the beach, where they share a passionate good bye with each other before Danbi disappears into the portal in the water. When she has disappeared, Lee swears that he will find her again, no matter how long it will take.
