Finding a nuke about to explode, Atom contains and absorbs the energy, apparently getting shunted back in time. A family takes him in temporarily and he assumes given the lack of any sort of tech that it’s the past, but when he sees the moon half exploded in the sky, he realizes something is up and then when the man he’s staying with tells him the year, he understands that he’s several hundred years in the future.
And that apparently superhumans caused all of this.
He makes a trip with the man’s elderly father to see a woman named Old Karrie, who knows him. Karrie is Kara Zor-L, Power Girl. So much kryptonite left her powerless and mad, but immortal, and before Atom’s powers send him home, she tells him flat out.
“Max Lord caused this.”