What in The World Happened?

A coffee shop owner, Il-woo and Nail artist, Anna are in a happy romantic relationship. They keep their relationship in a secret because of their age difference. One day, Anna's grandmother gives Anna a magic card. The card is to make her wish come true. After that day, she turns into a 10 years old girl, Hannah. Anna's wish "to become young"come true!

Il-Woo tries to find Anna since she disappeared suddenly. Il-Woo thinks Hannah as Anna's daughter, he takes Hannah to his house to take care of her. So they start to live together...As some time passes, Il-Woo finds out Hannah was Anna herself and got surprised, but soon gets used too it. On the other hand, Il-Woo and Anna help Anna's grandmother to find her first love.

Il-Woo and Anna help Anna's grandmother to find her first lover. With their help, Anna's grandmother finally meets her first lover. Grandma died testate not to lose her first love. By realizing the preciousness of love, Anna turns back into an adult. Il-Woo and Anna finally meet again and realize how precious they are for each other and make a promise for everlasting love.