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So-won, a high schooler, is about to turn 20 in a couple of hours. She meets someone and gets under a curse as she declines the mysterious person’s offer. On the day she turns 21, everything goes back to the time she turned 20. Now So-won has to figure out how to uplift the curse.

So-won goes to Hanhwa Resort to look for the woman who put her under the curse. And there, she runs into a boy who looks exactly the same as Young-seok. So-won decides to get a job at the resort and has an interview.

So-won joins Hanhwa Resort and starts to live in the dormitory, where she meets roommates of her age. So-won and her roommates go out for some drinks and run into Sang-uk and Kang-hyeon. There’s a rumor spread in the company that Sang-uk and Kang-hyeon are in a relationship.

Deadly drunk, So-won tells Kang-hyeon about the witch and even calls him Young-seok. Min-ji also runs into the witch after failing to get promoted. The witch tells Min-ji that she will help her if she buys a pack of gum from her.

So-Won runs to where the last wizard appeared to meet Team Leader Bong, but he disappears without a trace. Sang-Wook comforts So-Won who is devastated by the incident. So-Won tells Team Leader Bong that they are friends for 20 years. He returned to find the witch and chase her. But instead he was trapped in the Warehouse.

Kang-Hyun saves So-Won who loses consciousness. So-Won remembers her first love after seeing the warm side of Kang-Hyun who helped her. Kang-Hyun's ex-girlfriend meets her at the resort.

Kang Hyun is shaken by Noo Ri's suggestion to start over. Sang Wook confesses his feelings to So Won, but she runs away to avoid the fear of being hurt again.

Noo Ri had repeated the cycle of returning to Kang Hyun and leaving. Kang Hyun vows never to fall in love with her again. Noo Ri finds out Kang Hyun likes So Won and becomes jealous. The witch reappears and offers Yeon Ju, the resort staff, to make her family disappear.

During dinner, So-Won and Noo-Ri get into an argument. Kang-Hyun sided with Noo-Ri, not So Won. So-Won expresses her disappointment. So-Won hears about the witch and follows her. He finally met a wizard after 20 years.

So-Won is shocked to learn that the witch has Alzheimer's. The witch must lift the curse before she loses her memory, but instead she escapes from the hospital. At the resort, So-Won becomes sad when she meets her first love who is now a middle-aged man.

So-Won reunites with the witch. He was shocked to hear that he would be 40 years old if the curse was removed. Before the witch releases her curse, So-Won says goodbye to everyone and promises to return to Kang-Hyun.

Kang-Hyun misses So-Won but he can't find her anywhere. He finds a novel called My 20th Twenty written by Kang So-Won and is sure that it is So-Won. Kang-Hyun then looks for and meets again with So-Won who has been released from the curse.