the plot is mainly about a normal dude with a normal life but with a rather troubled past, if I may say, who has the opportunity to completely change his life through some kind of "event" he cannot really control, it just happens randomly, and has happened for much of his life, but never in such a "disconcerting" way
everything now is kind of a spoiler and I wouldn't recommend you to read if you are already interested enough
this "event" throws MC back in the past, whenever something "important" happens near him and he could had changed it, he will forcibly be sent back to fix things. He mentions it happened a lot of times in his life but short periods like 5 seconds ago.
Following this concept, the anime mainly runs through his childhood and his current life. In the present some stuff happens in his life and he is screwed, in the past he witnessed a tragedy and always felt a little responsible responsible. Later on he'll find oout both present and past tragedies causes are interconnected and he'll try to fix everything.
summing up it's a slice of life anime with an unexplained supernatural event attached to it. One of my favorites animes tbh