Season 1 is mainly about Lee Yeon's romance with Jiah and Lee Yeon mending his relationship with Lee Rang, his little brother. Lee Yeon also fought with Imoogi and lost his life but Lee Rang sacrificed his life in exchange for his brother's return. For more detail, start watching season 1 from scratch.
Season 2 is about Lee Yeon who came back to the past to get the stolen stone from Moo Young(Yeon's best friend), solve the demon's problems, resolve any misunderstanding with Lee Rang again, and go back to the future.
For people who asked, whether to watch season 1 first or just started with season 2 without 1, you can choose to do both.
- If you don't need to know about Lee Yeon's past or how his relationship with his brother works in the 1st season, then you can go ahead and start the 2nd season.
- If you want to know more about Lee Yeon and Lee Rang, then I suggest you finish season 1.
It is best to start from season 1 for people that haven't watched this drama. The first season explained a lot about the brothers' histories, especially, Lee Yeon's past will be shown a lot. You'll understand the plot in season 2 even better if you finish the 1st season.
I'm addicted to both seasons of this drama and fell head over heels for it. It is so good. I especially love the relationship between the brothers. So, I hope people who need clarification on how to watch this drama will find this useful.
Enjoy~