V zaklyuchitel'noy knige trilogii o Kate Ershovoy chitateley zhdut novye priklyucheniya uzhe polyubivshihsya geroev. V chetvyortyy klass shkoly pridyot novyy mal'chik, kotoryy, konechno, podruzhitsya s glavnymi deystvuyushchimi licami: YAsey Begunkovoy, Lyon'koy Starostinym, Daney Veselkovym, Dimoy Dumcevym i Artyomom Braginym. Vmeste oni poprobuyut napisat' knigu, vospol'zovavshis' sovetami nastoyashchey pisatel'nicy, pobyvayut v starinnoy Kolomne, spoyut pesnyu Vysockogo i stancuyut na vypusknom vechere, pomogut proyti pedagogicheskuyu praktiku Katinoy mladshey sestre Margarite, posetyat shokoladnuyu fabriku, vernutsya v proshloe, chtoby prozhit' odin den' v XIX veke, osilyat zabeg na tysyachu metrov i napishut pis'ma v budushchee.