Who saw the Time travel Netflix show, In the Shadow of the Moon -MAJOR SPOILERS

Intriguing, but I have a problem with a premise of Time Travel that is a major twist for the character's plotline.

Okay, here it is: A man's granddaughter's timeline intersects with the man's timeline in reverse order. In other words, she is traveling in time in reverse order. She gets older by a month each time they meet, while the grandfather grows old about seven or ten years each time. Keeping this pattern in mind, the first time he meets her is about her fourth or fifth jump backwards for the granddaughter. It is also a tragic meeting, because it results in her death. Despite this fact, they will continue to meet every ten years. She, in a younger version of herself, and he getting older chronologically in correct order.

To add to the complication of this relationship, the grandfather is a cop, and she acts like a terrorist, picking out youngish people for extermination on each jump. So he is determined to stop her, while she is on a mission like her life depends on it.

In this unlikely plot, the granddaughter knows who he is, but he has no knowledge of how they are related until the day of her actual birth. It's on that day that they meet face to face and she tells him everything, because she needs him to play the role that started her on her journey. She tells him that he loved her and taught her all the techniques that she needed to evade police. And she tells him that he joins her cause, which apparently is a good one. In her time a civil war is started by a white supremacist group that tears the nation apart, throwing everyone in chaos. So she is going back in time, eliminating the principals.

So, this is the part that confuses me. On that meeting, the last one before the day of her birth, when he is finally convinced that she is not an evil entity, he tells her that she has to be careful because she dies back in 1988 when he first meets her. She says that if it has already happened, she can't undo it. That makes no sense to me at all. She still hasn't jumped backwards at the older age, so why can't she undo it?

That's the time travel dilemma of the show. I can understand if she misleads her grandfather intentionally because she needs him to make every decision as it already happened.

The movie has a very narrow plot, with a human heart otherwise.