Premise:
Modern-day Korean salaryman transmigrates into the body of the second prince of the medieval proto-Korean kingdom. However, he's the younger brother of the most infamous tyrant in the history of the kingdom. Furthermore, he finds himself already married to a pretty, devoted girl, and he already knows that if the history he learned in high school would come to pass, the king would be overthrown, and he would be separated from his wife due to politics. To avoid this, he sets out to temper his brother's mood swings and make sure he doesn't end up as a hated tyrant.
Setting:
The Joseon Kingdom was the predecessor of modern Korea. It's a mostly historical setting, though due to the MC's meddling, history goes off the rails, and a lot of technologies get introduced way early, such as soap and vaccination.
Characters:
We have our easy-going MC, who's trying his best to care for the mental health of his brother and king, yet just as often ends up accidentally encouraging him. Said king is an erratic young man suffering from mood swings, Freudian issues, and if left unchecked, he'd inevitably turn into a tyrant who'd get overthrown. Other main characters include the MC's devoted young wife, the king's scheming concubine, and a young female doctor who may or may not have a crush on the MC.
Plot:
MC transmigrates into the past as a prince, has to figure out how to stop his brother from going crazy and getting deposed, he "invents" a lot of modern technologies to help the economy and the healthcare of the nation, all the while a lot of political drama is brewing in the background.
Overall:
This is a really dense series that's hard to get into, and even though it tries its best to explain all the historical minutia, it's fairly difficult for a non-Korean reader to understand what's going on in its first couple of chapters. Once the reader overcomes this hurdle, the rest of the story basically boils down to three things:
The historical drama surrounding the king, kingdom-building, and the MC's antics with his easily embarrassed wife, his devoted servants, and him hanging out with his brother. At the end of the day, I'd recommend this to people already familiar with Korean history, willing to learn about it through its infodumps, or just generally interested in stories about political drama.