more spoilers below
so from what I understood Kyungmin and Gwanghae conceive a child, and someone finds out and uses that to try to dethrone him. But Jeongwon falsely confesses that it's actually his child to protect his bro and the girl he has unrequited feelings for (rip). So they get exiled to Jeju Island by the king. She gets sick bc she misses Gwanghae and miscarries despite Jeongwon's constant care (big F).....Once Gwanghae becomes King they both come back and they get married and gets pregnant again w/ twins?.... Kyungmin does something that causes a time paradox bc she changed history ( I think she prevented her 8 year old self from time traveling) and she goes back to the present.... after completing high school she goes back to the past to be with Gwanghae and 4 years later he dies in her arms.... after his death she then goes back to the present and becomes a uni prof and marries another man and has a kid but eventually divorces (and her daughter is the protagonist of a sequel book of the novel lol) I think Gwanghae get's reincarnated as a teacher in the present too.
So I heard this was based on a novel and from the spoiler I saw it actually has a sad ending
spoilers
he actually dies in her arms at the end. But historically speaking the real gwanghae had a bleak ending too (died in a coup detat) so maybe the story followed real events