The story begins with an unusual friendship between Aziraphale, an angel, and Crowley, a demon. Both have been stationed on Earth for millennia and have developed a fondness for human civilization. Despite being on opposite sides of the celestial spectrum, they form an unlikely alliance when they learn that the apocalypse is scheduled to happen imminently due to the birth of the Antichrist. The child is supposed to bring about Armageddon when he comes of age, marking the end of the world as it is known. However, due to a mix-up at the hospital by the Satanic nuns tasked with handling the newborn Antichrist, the child is misplaced and ends up in the wrong family.

Adam Young, the real Antichrist, grows up in the small English town of Tadfield completely unaware of his true nature. Unlike the stereotypical harbinger of doom, Adam is a relatively normal and well-meaning child with a strong imagination and a tight-knit group of friends known as "The Them." His upbringing, free from the direct influence of either Heaven or Hell, allows him to form his own independent worldview. As Adam approaches his eleventh birthday, his latent powers begin to manifest, subtly reshaping reality to reflect his personal beliefs and desires.

Meanwhile, Aziraphale and Crowley, realizing the mix-up, embark on a frantic search to locate the true Antichrist before it is too late. Their efforts are complicated by the arrival of supernatural forces eager to see the apocalypse come to fruition. The Four Horsemen of the Apocalypse—War, Famine, Pollution (who replaced Pestilence after the discovery of penicillin), and Death—begin assembling, setting events in motion for the final battle.

At the same time, Anathema Device, a descendant of the prophetic witch Agnes Nutter, becomes aware of the approaching end times through a book of prophecies written by her ancestor. Agnes’s predictions, highly specific yet cryptic, provide guidance on how events will unfold. Anathema eventually crosses paths with Newton Pulsifer, an awkward yet well-meaning man who has inadvertently joined the modern-day Witchfinder Army led by Sergeant Shadwell. Despite their initial differences, Anathema and Newton play a pivotal role in deciphering the prophecies and understanding Adam’s true significance.

As the pieces fall into place, Adam’s powers fully awaken. Unlike the traditional apocalyptic figure, Adam does not desire destruction and chaos but rather a world that aligns with his vision of balance and harmony. When confronted with his true nature, Adam chooses free will over fate, rejecting both Heaven’s and Hell’s plans for Earth. He effectively unravels the impending Armageddon by using his powers to dismiss the Four Horsemen and halt the celestial war.

In the aftermath, Aziraphale and Crowley find themselves at odds with their respective superiors in Heaven and Hell, both of whom are dissatisfied with the failure of the apocalypse. However, through a clever ruse, the duo manages to avoid punishment, returning to their earthly lives with a renewed appreciation for humanity. Adam, having unknowingly saved the world, resumes his normal life, his powers fading away as he embraces his human existence.

With its witty dialogue, satirical commentary on good and evil, and endearing characters, Good Omens offers a refreshing and humorous take on the concept of destiny and free will. The novel blends elements of fantasy and absurdity, creating a compelling story that explores the complexities of morality and the unpredictable nature of the human experience.