This is a summary based on our understanding of the series.
Please also note that this summary contains spoilers.
Shim Cheong was a mermaid who fell in love with Joon Jae, who was a con artist. She followed him around since she’s very clueless with how everything works in the land. When they both fell in love, they discovered that they were reincarnated and in their previous life, they both died. They don’t want history to repeat so they kept protecting each other. When Shim Cheong got shot she came back to the ocean to regain her strength and came back to land to be with Joon Jae once again, happily.