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.