As one could have easily predicted, several members of the lunatic fringe managed to figure out how to make a viable atomic bomb from ingredients you might have in your kitchen. One guy decided to use his.
He didn't really have a goal, but he certainly had the means. What is the point of not using the means when you have them? So he did. Of course, in the absense of the Cold War and of any other super-power nations, most of the arsenal that the good ole USA had built in the defense boom of the 1980's were languishing in long since forgotten silos all over the Earth. Turns out, many of them were duds, funny enough. But the others were live, sure enough. And our friendly neighborhood psycho (AKA convenient scapegoat) dropped his in such a place that was central to a few other psychos, and low and behold, a chain reaction of chain reactions swept the Earth clean in a matter of days.
Relatively clean, anyway. In fact, only two animals survived the blasts, the fires and the winds. They were weak from the months of intense radiation, their eyes were still adjusting to the dawn that was just beginning to peek through the nuclear winter ash flakes swirling in grey-outs. One was a rat. The other, a snake.
This is not your average rat. He is the size of a small German Shepard.
The snake is the deadliest snake you can imagine. Some believe it is a poisonous snake, with acid fangs and a whip's speed. Others choose a power snake, like a boa, just one long thick muscle that wraps and wraps and then squeezes, slowly.
It all comes down to this: Rat vs. Snake. One-on-one. To the death.