To very briefly summarize for a quick read on whether or not you should watch this drama (without spoilers):
Kairos seemed like something I would enjoy: a man who fights to save his daughter and wife through the use of time travel communication. Unfortunately, as the story progressed, the story got more ridiculous. There were so many plot conveniences that were just logically dumb; the wife's plot and the severe character change in the secretary - to briefly name a few without spoiling anything. The decisions these characters make just don't make sense, even if you look from their emotionally driven perspective (PLOT CONVENIENCES UGH). Personally, don't waste your time with this drama. It's really not worth your mental sanity - it will leave you frustrated and exasperated.

A detailed breakdown of why you shouldn't watch this drama to clarify my brief summary above since this drama is getting a lot of attention (spoilers below):
I initially started to realize I didn't like this drama when it was revealed that the wife is mentally unhinged and cheating on our MMC, Kim Seo Jin. What made this so unpleasant was that the story wasn't written this way initially. To the viewer, she has no logical reason to cheat on her husband; while he is a workaholic, he loves his daughter and takes care of his wife. They took it further by the wife faking her own suicide AND the faking the kidnapping/horrific MURDER of their daughter. They then try to justify this horrors by making her this villain with sociopathic tendencies, which was unsettling and not the type of drama I sought out to watch.
***Predictable - of course the wife is a cheating bitch, of course the daughter is alive. *rolls eyes*

Then there's the secretary that they did a similar 360 character change. He was a bit fishy in the beginning and his ploy was predictable, but his severe character change to this heartless villain that adjusts his glasses all deviously and has a cold tone of voice just makes no sense from the type of character he was prior to his ploy reveal (the change is too drastic - it feels unnatural).

Then I just couldn't wrap my head around the disabled father who lost his own daughter in an accident. His goal was to get revenge on Kim Seo Jin because he had been convinced that KSJ is responsible for his daughter's death. Okay, sure, I can see how that happened. But then he turns around and uses KSJ's daughter, convincing KSJ that he's kidnapped and murdered the little girl?! What type of father, who he himself is grieving the loss of his child, could bring himself to commit such atrocities?! And while some people might be able to justify it saying, "he wanted KSJ to feel what he felt," I just can't stomach it.
***Predictable - of COURSE the disabled father was going to be killed when it appeared like he was going to reveal the plot.

Then there's the male "friend" to Han Ae Ri who stole her entire life's savings meant to pay for her mother's heart transplant surgery. She MURDERED him for what he did in the original timeline! While I understand her not killing him in the new timeline (which I can imagine was really hard considering the situation), how can they just be friends again? She was guarded at first, but now it seems like they trust and depend on each other and it just doesn't add up to me. If that was me, I wouldn't be able to. *shakes head*

The phone calls between KSJ and HAR were just ineffective. I know that they only had a minute, but it seems like *they* didn't realize that. *rolls eyes* Logically, it would make more sense to send a bunch of text messages to be delivered in that one minute so they have EVERYTHING they need for the day rather than these constantly interrupted phone calls.

Also, I know this doesn't matter in the grand scheme of things, but it was so distracting: who decided on the awful hair cut/style for LSY's character? It looks so unflattering - in everything else I have seen her in, her hair is so beautiful, and SHE is so beautiful - just WHY?!
And could the writers stop making HAR cry about things that haven't technically happened yet? KSJ is telling her what to do to PREVENT these terrible things from happening, so stop freaking out before anything has happened! Be relieved and motivated that there's something you can do to stop this. UGH.

To summarize:
Why add all of the amazingly acted, choreographed, and portrayed theatrics of agony in the first 2 episodes if it was going to be changed to a revenge plot? The first two episodes made Kairos appear like a drama about a man who loses his entire family - first his daughter, which breaks him and his wife, then his wife. He becomes so driven to save them once he realizes he might be able to change what happened through the help of HAR. But then the plot devolved from a drama about a man saving his family, to a convoluted revenge plot with over-the-top, unrealistic villains (based on how they were written and portrayed in the beginning). Would it have been so bad if this drama was unique from other dramas of similar genre where the wife was dedicated and loved her husband and they had a normal relationship? To create *shock* value, this drama loses substance through the use of these "plot twists." I just couldn't watch anymore of this nonsense. Too many things that were incorporated in the plot to accommodate the progression of plot that I just couldn't tolerate anymore. In conclusion, I was expecting a really different plot from what was delivered, and this, along with the mentioned flaws, is why I am dropping it.