OK. let's talk about a series of episodic stories, specifically Joss Whedon's Buffy The Vampire Slayer. We'll take a look at Season One because if you are going to write a series, you will start by writing a 'Season One'...a group of stories that introduce the main characters and their situation and get the plot rolling. Season One will have an overall arc (we'll call it the A Story) that will take the entire season to be resolved.
It will also have subplots (we'll call these the B stories) that run over multiple stories.
Each story itself will be in MOTW (Monster Of The Week) format and we will call it the C story.
This is a good time to throw out a definition. What we are talking about in this blog is a series of stories, not a serialized novel. A serialized novel is a novel that is released in chunks. What we are talking about is a series where every story (episode) is complete in itself. Every story will have a C story and the C story (the MOTW story) will be resolved in that story. If the difference between a serialized novel and an episodic series isn't clear, leave a comment or email me and I will post more detail on the difference between the two.
So, to summarize....
A Story. Arcs over the entire series. Usually involves the most dangerous enemy to the hero.
B Stories. Arcs over many episodes. Usually sparked by the personalities of the main characters. Possibly romantic entanglements.
C story. The Monster Of The Week. Resolved in each episode. Does not arc.
Buffy The Vampire Slayer by Joss Whedon
Written by: Joss Whedon, Steven S DeKnight, Jane Espenson, David Fury, Drew Goddard, Drew Greenberg, David Greenwalt, Rebecca Rand Kirshner, Marti Noxon and Doug Petrie.
Episode 1 & 2: Welcome to the Hellmouth/The Harvest
Synopsis: Buffy Summers arrives at Sunnydale and we learn that she is The Slayer, a chosen one destined to fight vampires and demons. She meets Giles, (her new Watcher) and Willow and Xander. Through Giles, Buffy learns of a coming Harvest led by a strong vampire called The Master. He is trapped in an extra-diensional space but is working toward his own release.
A Story. The situation with The Master is set up. The Master is the'big bad' and Buffy's fight against him will not be resolved until the last episode of the series.
B Stories. Xander is attracted to Buffy but she is attracted to the mysterious Angel. Willow has been in love with Xander since they knew each other as kids but she has never told him.
C Story. Willow is taken by vampires. Buffy saves her and fights other vampires throughout the episode. At the end of the episode, she kills Luke, a strong vampire minion of The Master.
So here we have everything set up. We know who Buffy and her pals are and what their relationships to each other is. We know about The Master and we have had some Buffy action and fighting to tie up the C Story.
As for the characters, Joss Whedon has set this up so that the characters are all very different in outlook and personalities.
Buffy Summers. Strong, kick-ass vampire slayer.
Willow Rosenberg. Meek and mild bookish girl. (although she arcs in later seasons and changes)
Rupert Giles. Watcher from The Council. Fatherly and protective toward Buffy. Knowledgable in occult matters.
Xander Harris. Joker and clown with a crush on Buffy.
Angel/Angelus. Vampire who works on the side of good due to a gypsy curse that has given him a soul. Loves Buffy.
Cordelia Chase. Spoilt and rich girl. Vapid and opposed to 'geeks'. (Willow, Xander and Buffy)
So you can see that the series is built on fertile soil.
I suggest you watch Buffy and see how the writers create stories from the initial set up. You will see how clues are laid down in certain episodes that will become realized later in the season. (In a later season, a clue is laid down that isn't realized until the following season.)
If Buffy isn't your thing, watch your favorite show with the same analytical eye.
Then think about your own series, get a notebook and think about these things:
What is your A Story?
What are your B Stories?
What are your C stories?
Who is your main character?
Who are your supporting characters and what about their personalities/quirks will drive the B Stories?
What situation are your characters in that will make them interact/work together throughout your series?