The Yooks and Zooks live on opposite sides of a long curving wall. The narrator of the story is a Yook child whose grandfather takes him to the wall, explaining he is a retired soldier. The Yooks wear blue clothes, but the Zooks wear orange. The difference between the two cultures is that while the Yooks eat their bread with the butter-side up, the Zooks eat their bread with the butter-side down. The conflict between the two sides leads to an escalating arms race, which results in the threat of mutual assured destruction.