I see the series as a mashup of Terminator, Back to the Future, Minority Report and Life on Mars. I get that it posits the future cannot change, just like the rock Sisyphus pushes up the hill will always falls down again, no matter how many times he pushes it up again. Even when the story seemed to finally change the future (for the good), the final scene with Sigma suggests not.

I gave high marks for the story and acting, but with serious reservations. Too many times I wanted to shout out "Take the shot." Too many times the Gang Seo-hae character stood there like a deer in a headlight, while normally she performed admirably when in action. Too many times she cried too much and too long. And the dialog sometimes didn't make sense.

The series could have been completed in 12 episodes instead of 16. I kept wanting to fast forward through scenes that dragged.

Since the actors were good and the plot was clever, I blame what I consider flaws mostly on poor direction but also on poor writing.