In a world where everyone is a cereal mascot and cereal is grown on farms, Penn seeks the help of an imprisoned Rippen to catch The Milkman, a villain threatening to flood all the cereal crops with milk, turning them soggy.
This is the first episode in which Rippen isn't the main villain, as the Milkman is instead.
The brief appearance of "Lenny the Lion" is likely a reference to Linus the Lion-Hearted, a 1959 cartoon created to advertise various Post cereals such as Alpha-Bits, Crispy Critters, Post Toasties, Rice Crinkles, and Sugar Crisp; Linus himself was voiced by well-known character actor, Sheldon Leonard. (Get it?) In addition, the Alpha-Bits mascot, a friendly, Southern-accented Postman, Lovable Truly, is very similar in depiction to The Milk Man in this episode.
This episode reveals Rippen and Larry's jobs as an art teacher and school principal respectively.
The title refers to the time-honored pun of "cereal" ("derived from or relating to grain") and "serial" ("referring to a number of actions or events that happen in a particular time-sequence"); a "serial criminal" is one who commits a number of similar unlawful actions over a certain period of time.