An unnamed father and his young son journey across a grim post-apocalyptic landscape, some years after an unspecified disaster has destroyed civilization and most life on Earth. The land is filled with ash and devoid of living animals and vegetation. Many of the remaining human survivors have resorted to cannibalism, scavenging the detritus of city and country alike for flesh. The boy's mother, pregnant with him at the time of the disaster, gave up hope and committed suicide some time before the story began, despite the father's pleas. Much of the book is written in the third person, with references to "the father" and "the son" or to "the man" and "the boy."
Realizing they cannot survive the oncoming winter where they are, the father takes the boy south, along empty roads towards the sea, carrying their meager possessions in their knapsacks and a supermarket cart. The man coughs blood from time to time and eventually realizes he is dying, yet still struggles to protect his son from the constant threats of attack, exposure, and starvation.
They have a revolver, but only two rounds. The boy has been told to use the gun on himself if necessary, to avoid falling into the hands of cannibals. During their trek, the father uses one bullet to kill a man who stumbles upon them and poses a grave threat. Fleeing from the man's companions, they have to abandon most of their possessions. As they are near death from starvation, the man finds an unlooted hidden underground bunker filled with food, new clothes, and other supplies. However, it is too exposed, so they only stay a few days.
In the face of these obstacles, the man repeatedly reassures the boy that they are "the good guys" who are "carrying the fire". On their journey, the duo scrounge for food, evade roving bands, and contend with horrors such as a newborn infant roasted on a spit and captives being gradually harvested as food.
Although the man and the boy eventually reach the sea, their situation does not improve. They head back inland, but the man loses blood after being shot with an arrow. He dies, possibly due to his long-standing respiratory ailment. The father tells the boy that he can continue to speak with him through prayer after he is gone. The boy holds a wake over the corpse for a few days, with no idea of what to do next.
On the third day, the grieving boy encounters a man who says he has been tracking the pair. The man, who is with a woman and two children, convinces the boy that he is one of the "good guys" and takes him under his protection.