Good Omens S01E01 - In the Beginning

Two celestial beings, the angel Aziraphale and the demon Crowley, have gone native and worry when their superiors tell them to prepare the Antichrist for Armageddon. They resolve to secretly join forces to make sure the Antichrist is neither good nor evil... unaware that there was an unwitting switch made and the Antichrist isn't who they think.