A college student, Crain, explores the post-apocalyptic world with his old dissertation advisor, Dr. Ormon. The two, who both studied anthropology, follow a horde of zombies that walk along a freeway to nowhere. Dr. Ormon believes that the zombies hunt using sound vibrations and so they attempt to stay off the asphalt roads. Using their anthropological knowledge, they also follow behind the horde and eat the marrow from the bones of the humans that the zombies have devoured. 

As they walk they discuss the evolutionary steps that humans have gone through in order to reach their present selves. They discuss the idea of the first bipedal mammals and their ability to become persistence hunters. They compare their own actions to the actions of the zombies ahead of them and hope that the zombies do not learn to become persistence hunters or how to extract the marrow from the bones they are eating. Crain believes that he has outsmarted his professor as they discuss the future of Crain’s hypothetical dissertation, specifically what he will write in chapter six about the history of humans being persistence hunters.

The next day they come across a group that has recently been killed by the horde of zombies and pick the bones for marrow. They hear a baby crying inside a backpack and Dr. Ormon goes to kill the baby — predicting a risk by keeping it alive. Crain seems uncomfortable with this decision. However, the baby's cries attract a single zombie from the back of the horde which Crain kills using a tibia. Crain breaks off the zombies arms and gives the marrow to Dr. Ormon, who eats it and begins to turn into a zombie himself. Crain renounces the way of marrow sucking as Dr. Ormon dies. A cry from the backpack indicates that the baby is still alive, who Crain pulls out of the backpack and names Eve. He decides to head into the trees.