The Curious Condition of Being Human
For a time, the forces of Heaven and Hell left Aziraphale and Crowley alone. But with new plans in the works, the forces of good and evil want to make sure the meddling pair stay out of the way. They are still afraid of the newfound powers the pair seemed to possess, as demonstrated at their trials. So instead of coming at them directly, Gabriel and Beezlebub marry celestial and infernal curses to make Crowley and Aziraphale think they're human.
But their plan, as usual, is imperfect. Crowley's enchantment fails almost immediately, but Aziraphale is still under the delusion that he is quite human indeed. Crowley needs to try and undo the curse without drawing the attention of celestial forces who might correct their enchantment and try a second time.
