Good Omens is the story of an angel and a demon who are both trying to do their jobs that are part of the Great Plan. Although Aziraphale and Crowley are adversaries by nature and profession, their relationship develops into a friendship merely because of the time spent together over thousands of years. In this unlikely pairing, compromises are made between the two of them so that they can both appear to be accomplishing their missions without overcoming the other too much. When the birth of the Antichrist occurs, they agree to work together and try to see if their influences on the child have any effect. By the time they locate the correct child, it is almost too late as Armageddon is about to begin. Events unfold and the world is saved. At the end, Aziraphale and Crowley wonder if their involvement had any effect.

From the beginning, it is clear that Aziraphale and Crowley are both instigators in their own way; their instigations having an enormous effect on the course of events in the world. After the birth of the Antichrist, Crowley convinces Aziraphale that they should do what they need to in order to influence the child's upbringing. They appear in the child's life as the nanny and the gardener, as well as other characters that will place them near the child. Panic sets in, however, as they discover they have been influencing the wrong child. Due to a baby-switch gone awry when the Antichrist arrived at the hospital eleven years prior, they have been spending their time and energies on the wrong child.

Meanwhile, young Adam and his friends are enjoying the summer as normal eleven-year-olds would. Through an interesting turn of events, Adam meets his neighbor, Anathema, who is a self-proclaimed witch. When she passes magazines along to him, his nighttime reading creates real-life havoc. It is these strange events that catch the watchful eye of Newt Pulsifer, a Witchfinder Detective in Witchfinder Sergeant Shadwell's Witchfinder Army. As Newt investigates the strange goings-on, he decides to pay a visit to Tadfield, which seems to have its own unusual story: perfect weather all year round. After a minor car accident brings him to Anathema's doorstep, he is further pulled into the realm of the supernatural. Anathema explains about Agnes Nutter's Prophecies and how the end of the world is about to happen.

Bent on preventing Armageddon, everyone converges at the Tadfield Air Base. Adam, who has some unusual issues going on within his own mind, convinces his friends to get their bikes and pedal along with him. Crowley speeds along in his Bentley while Aziraphale possesses the body of Madame Tracy. With Shadwell in tow, she makes the journey on her moped. Newt and Anathema follow Agnes Nutter's Prophecies and also follow suit. Once they are all there, Adam surprises everyone present by saying that he sees no reason why the world needs to be blown up again only to have the same cycle repeat itself. Somewhat disappointed, the Four Horsemen/Bikers of the Apocalypse, as well as the Metatron and Beelzebub, and Anathema depart. Everyone else breathes a huge sigh of relief.

The next day, no one is able to recall clearly the events of the previous evening. Even as Aziraphale and Crowley debate whether or not the events were part of the divine plan or not, their memories are wiped out as well. They continue on as they had before the Antichrist entered the picture.