In an alternate history, the asteroid that would have caused the extinction of the dinosaurs 65 million years ago passes safely over Earth. Millions of years later, Apatosaurus corn farmers Henry and Ida have children: Libby, Buck, and the runt, Arlo, who has trouble adjusting to farm life. While his successful siblings are allowed to “make their mark” (a mud-print on the family’s corn silo), Arlo’s timid nature makes tasks difficult for him. Henry attempts to give Arlo a sense of purpose by putting him in charge of guarding their silo, and helps him set a trap. It captures a caveboy, but Arlo cannot bring himself to kill him, and sets him free. Disappointed, Henry takes Arlo to track the caveboy, leading them into a ravine. Henry saves Arlo from a flash flood before being swept away and killed.