The demon serpent Crowley and the angel Aziraphale sit outside of the Eastern Gate. Crowley thinks it’s silly that God expelled Adam and Eve from the Garden of Eden for eating from the Tree of Knowledge—and Aziraphale, being thoughtful and generous, admits that he gave his flaming sword to Adam and Eve so they can protect themselves.
After this, Crowley and Aziraphale spend the next 6,000 years on Earth and come to love it. They also form a close friendship. Because of this, Crowley is distraught when he receives word that Armageddon (the end of the world, which will user in a war between Heaven and Hell) is upon them. It’s up to him to deliver the Antichrist (the son of Satan who will usher in Armageddon) to a hospital to be switched with a human baby. After completing this task, Crowley alerts Aziraphale. Neither can stand the thought of either Heaven or Hell winning the final war, so in the back room of Aziraphale’s London bookshop, they decide they’ll try to stop it from happening by intervening in the Antichrist’s childhood. They reason that if he doesn’t get a true Satanic upbringing, he might not turn out evil and therefore might not end the world in 11 years.
Unbeknownst to everyone, though, there was a mix-up at the hospital. Instead of placing the baby Antichrist with the American Cultural Attaché and Mrs. Dowling, Sister Mary (a Satanic nun working at the hospital) accidentally places the Antichrist with Mr. Young and his wife Deidre, who live in the rural English village of Tadfield. As the Antichrist, Adam, grows up, he becomes the ringleader of a gang of kids known as the “Them.” Meanwhile, the boy whom Aziraphale and Crowley think is the Antichrist, Warlock, seems far too normal—and on his 11th birthday, he doesn’t receive a hell-hound like he’s supposed to. Adam, however, does receive a dog for his birthday. Out of Adam’s sight, the massive, slavering hell-hound becomes exactly the dog Adam wants: a small, scrappy mutt. Adam names the dog Dog, and the animal starts to become less evil in his new form.
Realizing that there’s been a mix-up, Crowley and Aziraphale search for the hospital where the switch took place over a decade ago. On the drive there, Crowley hits a young witch named Anathema Device with his prized vintage Bentley. Anathema is the descendent of a 17th-century witch, Agnes Nutter. In addition to being a witch, Agnes could see the future and published The Nice and Accurate Prophecies of Agnes Nutter, Witch. Anathema owns the only copy in existence, but she accidentally leaves it in Crowley’s car when he drops her off at home after the accident.
When Aziraphale and Crowley are unable to find any useful information at the former hospital (which Mary has since converted into a corporate training center), they agree to call on their human networks for backup. Unbeknownst to them, their respective “networks” are actually the same single person: Mr. Shadwell. Shadwell supposedly runs an organization of witch-finders out of London—but in actuality, he only employs one person, a young man named Newt.
Meanwhile, Anathema is distraught when she realizes that she lost her copy of The Book. Agnes predicted that Armageddon would arrive mere days from now, and Anathema is left with only her notes. She’s currently living in Tadfield, which she believes will be the site of Armageddon—and one day, Adam comes across her crying outside of her cottage. He’s excited to meet a witch. Anathema graciously sends Adam home with a number of occult magazines, which Adam devours. Aziraphale, meanwhile, is thrilled when he discovers The Book in Crowley’s backseat. As a rare book collector, Aziraphale has hoped to see this book for years. For a full day, he’s completely absorbed in reading The Book.
Adam begins to tell the Them about various things he read about in Anathema’s magazines, like the disappearing rainforests, the ills of nuclear power plants, and the Hollow Earth Theory. As he explains these things, odd phenomena begin to happen all over the world. The material in a nuclear reactor mysteriously disappears; multiple people spot a flying saucer and aliens; Atlantis rises from the sea; and South American rainforests multiply. As this is happening, a deliveryman makes several deliveries across the world to the Four Horsemen of the Apocalypse. He delivers a sword to War, a set of scales to Famine, a silver crown to Pollution, and a message to Death. The Four know that their time has come—Armageddon is imminent.
Meanwhile, Newt’s boring job as a witch-finder suddenly becomes interesting when he discovers that Tadfield has uncharacteristically normal weather. He convinces Shadwell to let him go and investigate—but when he arrives in Tadfield, he swerves to avoid a Tibetan man coming up out of a hole and crashes his car. Anathema tends to his injuries and tells him that Armageddon will happen in just a few hours.
Concerned for Newt’s safety, Shadwell decides to call on Aziraphale for help. But when Shadwell gets to Aziraphale’s bookshop, he catches Aziraphale communicating with the Metatron (the voice of God) and saying that he has it all figured out—Armageddon doesn’t have to happen. Believing that Aziraphale is a demon, Shadwell attempts to exorcise him and accidentally succeeds in sending him to Heaven. He also sets Aziraphale’s bookshop on fire.
Crowley, meanwhile, refuses to join the ranks of demons in Hell in preparing for the final battle; he seeks out Aziraphale instead. When he arrives at Aziraphale’s burning bookshop, he discovers only The Book and Aziraphale’s notes. After reading the notes, Crowley decides he has nothing to lose—he has to try to stop Armageddon. To get to the Tadfield air base, he drives his Bentley over the M25, which is shaped like an evil sigil and therefore kills anything mortal that tries to pass it. The Bentley catches fire, and Crowley has to use all his powers as a demon to pretend that the car still works for the rest of the drive.
While this is happening, Aziraphale enters the body of Madame Tracy, Shadwell’s neighbor, and asks for her help in stopping the apocalypse. When Shadwell gets home and recognizes Aziraphale’s voice coming out of Madame Tracy’s body, he finally agrees to sit and listen to what Aziraphale has to say. Shadwell agrees to kill the Antichrist, so the three of them take Madame Tracy’s motor scooter to Tadfield.
As Crowley, Aziraphale, Shadwell, Madame Tracy, and the Four Horsemen of the Apocalypse make their way to the American air base in Tadfield, strange phenomena continue to happen. All the while, Adam becomes increasingly distraught. Seemingly aware of his power as the Antichrist, he divides up the world between his friends and tells them that it would be best to burn the world down and start again. However, this only frightens his friends. Finally, when Adam sees how afraid his friends are, he screams—and a mysterious force seems to leave him. He decides that they have to stop Armageddon: armed with a makeshift sword, crown, and scales, they join everyone else in heading for the air base.
At the air base, the Four Horsemen are busy tampering with electronic and computer systems. Electricity goes out worldwide, and the world seems on the brink of nuclear war when the Horsemen emerge from the building and come face to face with Adam and the Them. As Crowley, Aziraphale, Madame Tracy, and Shadwell watch, Adam coaches his friends through vanquishing War, Pollution, and Famine. Then, Death salutes Adam and disappears. Newt and Anathema have been hiding inside the building, and Newt manages to break the computer system. Suddenly, computers and phone systems all over the world start working again.
Beelzebub and the Metatron eventually appear to appeal to Adam. They insist that Armageddon must happen, but Adam refuses to give the go-ahead to start the war. With Crowley and Aziraphale’s help, Adam proposes that it may be part of God’s plan that Armageddon shouldn’t actually happen. The Metatron and Beelzebub return to Heaven and Hell, respectively, to await instructions—for the moment, it seems like everything is going to be okay. But then, Crowley and Aziraphale sense that Satan himself is going to burst through the ground to punish Adam. Aziraphale (armed with War’s sword, which is actually Aziraphale’s flaming sword), Crowley, and Shadwell advance. But with a wave of his hand, Adam causes something to change: at the spot where Satan was going to burst out of the ground, Mr. Young instead arrives to shout at his misbehaving son.
The next day, Crowley and Aziraphale discuss whether or not this was all part of the ineffable plan. They also talk about the fact that Adam put things back to normal, but with tweaks: Crowley’s Bentley is back to its former mint condition, while Aziraphale’s bookshop now contains first editions of children’s books instead of misprinted Bibles. Newt and Anathema receive a mysterious package that turns out to be Agnes Nutter’s next book of prophecies, though Newt convinces Anathema to not dedicate her life to the book. Madame Tracy and Shadwell decide to get married and move to the countryside. Adam, meanwhile, ignores the fact that he’s grounded after yesterday’s events and sneaks out of the backyard with Dog. He steals a neighbor’s apples and thinks that apples are always worth getting in trouble for.
