It's a story that takes place in an apocalypse. Just cuz they aren't called Zombies doesn't mean it's somehow original.

That surgery was the only hope humanity had and that stinking drunk only wished to do right by his daughter. That brat was just his 2nd chance that he clung to, not an actual relationship between two peoples but one between a man and his insane desire to rectify his mistake. It's a story that is told in every apocalypse + Western movies + any anime you can think of.