Wrong Exit

When Grace, Adam and Ruby get lost in the fog they stumble into a parallel world, similar to ours, but one where a terrible catastrophe has occurred. A flood has destroyed half the country and there is a single survivor, Dave in the town they stumble across. Grace and her siblings have to work out a way home, while Dave must try and see if there is anyone left alive. When they part, they tell Dave the way back into their world, so should he need to, he can find them too.