His girlfriend died one day getting hit by a Corvette C8 he later heard from the news. So he took 27 years researching time-travel and finally he have developed a way to go back in time.
So after creating the way to go back in time he came back to the past in his highschool life, when they were still junior high. But suprisingly everything does not go to the way it was supposed to be, because they were both supposed to be childhood friends.
And now the supposed childhood friend doesn't know him. Worse another man was her childhood friend instead of him, so that means the promise that they made in the past about getting married when they were still children was broken as he was not the man to be the childhood friend of the girl before him.
Then he later learnt that the object that he created to go back in time was not a time-travelling device, but a device that lets him travel to the timeline where his girlfriend didn't meet him as a childhood friend but another one.