Grandma Thora is a poor cook. When she decides to bake cookies for a bake sale, Arthur and D.W. try to think up ways to stop her bad cooking from ruining it. Arthur decides to buy all of her cookies before anyone else can and hide them. But when Mr. Morris is injured by the cookies hidden in the janitor's closet, Arthur and D.W. have to tell the truth. As it turns out, Grandma Thora is not upset because she knows she is still a great family member.