I want to say this drama is a time travel mystery but it's actually not. It's an expertly carved puzzle that awaits the viewers to piece together to reveal the beautiful picture that it contains. If you rewatch the drama (which I'm doing right now), you'll realize that the show has given you all the clues in the earlier episodes. If you are a diligent viewer, your theories on what had happen, or what will happen, will most likely hit the mark (couple of my theories pan out. yay!). You keep watching, not because you want the official answer, but because you are so invested in the characters themselves that you can't bear the thought of not rooting for them to the very last second. And "Someday or One Day" is awesome from beginning to end.