A father and his young son journey on foot across the post-apocalyptic ash-covered United States some years after an undefined extinction event; the death of all plant life and virtually all animal life.[7] The boy's mother, who was pregnant with him at the time of the disaster, has died by suicide at some point after his birth.
Realizing they cannot survive the winter in northern latitudes, the father takes the boy south along county roads towards the sea, carrying their meager possessions in their knapsacks and a supermarket cart. The father is suffering from a cough. He assures his son that they are "good guys" who are "carrying the fire". The pair has a revolver, but only two rounds. The father has tried to teach the boy to use the gun on himself if necessary, to avoid falling into the hands of cannibals.
They attempt to evade a group of marauders traveling along the road but one of the marauders discovers them and seizes the boy. The father shoots the marauder dead and they flee the marauder's companions, abandoning most of their possessions. Later, when searching a mansion for supplies, they discover a locked cellar containing people whose captors have imprisoned them alive in order to eat them limb by limb and flee into the woods.
As they near starvation, the pair discovers a concealed bunker filled with food, clothes and other supplies. They stay there for several days regaining their strength and then carry on, taking supplies with them in a cart. They encounter an old man with whom the boy insists they share food. Farther along the road they evade a group whose members include pregnant women, and soon after they discover an abandoned campsite with a newborn infant roasted on a spit. They soon run out of supplies and begin to starve before finding a house containing more food to carry in their cart, but the man's condition worsens.
The pair reaches the sea, where they discover a boat that has drifted from shore. The man swims to it and recovers supplies, including a flare gun, which he demonstrates to the boy. The boy becomes ill. When they stop on the beach while the boy recovers, their cart is stolen. They pursue and confront the thief, a wretched man traveling alone. The father forces him to strip naked at gunpoint and takes his clothes together with the cart. This distresses the boy, so the father returns and leaves the man's clothes and shoes on the road, but the man has disappeared.
While walking through a town inland, a man in a window shoots the father in the leg with an arrow. The father responds by shooting his assailant with the flare gun. The pair moves further south along the beach. The father's condition worsens, and after several days he realizes he will soon die. The father tells the son he can talk to him after he is gone, and that he must continue without him. After the father dies, the boy stays with his body for three days. The boy is approached by a man carrying a shotgun. The man tells the boy he and his wife have a son and daughter. He convinces the boy he is one of the "good guys" and takes the boy under his protection.
