This story takes place after the events of the episode Lesson Zero

After taking the Smartypants doll that made everypony go crazy. Big Macintosh woke up the following morning to find out he was regressed to a baby. Not only that it seems that nopony has noticed that he was regressed in age and act as if he was always a baby. But he does have an idea that the Smartypants doll might be involved.