The story opens explaining how Brother and Sister Bear, the main characters, usually get along really well by waiting their turns to use the bathroom, using good table manners, sitting together on the bus, sharing toys and respecting each other well. But one gray morning, Brother and Sister's argument begins and quickly turns into a fight between the two of them, and eventually a battle between both of them. It is first started where Sister lets her feet dangle down in front of Brother's face, annoying Brother. Sister puts her head close to Brother in an annoyed look. Then, Sister, agitated, races off to the bathroom before Brother does and when she arrives at the bathroom taunts Brother. When Sister takes too much time in the bathroom, Brother angrily bangs on the door and shouting at her to get out of the bathroom. Papa comes out of his bedroom and says to Brother that this is no way to talk to Sister ("You know better than to shout at your sister!"). Brother says (complaining that Sister is hogging the bathroom by taking a long time in there), "But she's taking too long and she's doing it on purpose!". Sister Bear was really hogging the bathroom. Then, they use bad table manners at breakfast, sit in different seats on the bus (Sister Bear sits in the front and Brother Bear sits in the way back), take back the toys they normally shared (Sister takes back her modeling clay that Brother made into dinosaurs for his school project and rolled it into a big lump; while Brother hides his toy planes and trucks), and draw a line down their backyard tree house to split up, determining which side is their own. After Mama and Papa Bear get fed up with their shouting, Mama Bear (after whistling loudly at Papa, Brother, and Sister from shouting) explains how everyone gets in arguments sometimes, and remarks however Brother and Sister probably forgot what caused them to fight at all. And the next day, they make up with each other and apologize, and wash off the line they have drawn down the backyard tree house until the next time, anyway.