Hundreds of cash-strapped players accept a strange invitation to compete in children's games—with high stakes. But, a tempting prize awaits the victor.

How can a mermaid from the Joseon era survive in modern-day Seoul? Shim Chung is a mermaid who finds herself transplanted to modern times. She is caught by Heo Joon Jae, a charming but cold con artist who is the doppelgänger for Kim Moon, the son of a nobleman from the Joseon Dynasty. But in the present time, Joon Jae works with Jo Nam Doo, a skilled conman who guides Joon Jae to become a genius scammer. But Joon Jae’s friend, Cha Shi Ah, who works as a researcher at KAIST, may be Chung’s only hope for surviving in her strange new world.

On the eve of the founding of the Joseon Dynasty, a whale swallows the Emperor's Seal of State being brought to Joseon by envoys from China. With a big reward on whoever brings back the royal seal, mountain bandits led by Jang Sa-jung go out to sea to hunt down the whale. But he soon clashes with Yeo-wol, a female captain of pirates, and unexpected adventure unfolds.