The Butter Battle Book tells the story of a land where two hostile cultures, the Yooks and the Zooks, live on opposite sides of a long curving wall. The Yooks wear blue clothes; the Zooks wear orange. The main dispute between the two cultures is that the Yooks, who live on one side of the wall, eat their bread with the butter-side up, while the Zooks, who live on the other side, eat their bread with the butter-side down. The conflict between the two sides leads to an arms race, each competing to make bigger and better weapons to outdo the other, which results in the threat of mutual assured destruction.