When Mildred's attempts to revise her potion test notes fail, Mildred & Maud produce an invisibility potion by mistake, getting the duo into trouble! Mildred is determined lie low in an effort to stay out of trouble, but the Hallowe'en festival looms, & Mildred's class has been chosen to put on a flying display! A very cross Ethel decides to sabotage Mildred's broom!