I started structuring out some plots today for the serial. Since I'm planning to use 100 page novelettes, I decided to use movie structure, which is really suited to that length and especially suited to the serial concept. Serials are great with formula, and movie structure is so flexible, it actually fits any type story you want anyway.
The overall book will be four novelettes, each with a complete story, but all tied together with a bigger story. Each novelette will have four acts - almost approximating a movie structure. Standard Hollywood movie structure is described as three acts, but the middle one is twice as big as the others, and has two parts, so really it's four acts. (The problem is that Syd Field, the first guru of movie structure, didn't recognize the importance of the mid-point, so the terminology in Hollywood is stuck with the original three-act terminology. When you say "third act" you mean the end, no matter how many acts you have in your screenplay structure.)
Luckily, I'm not writing a screenplay, so I don't have to mess around with "Act 2 part A" or anything silly like that. I can just have four acts. Or more if I want. For those who might find it interesting, here is a quick description of classic movie four-act structure. Some of this comes from my screenwriting hero, the late Blake Snyder, and some from observation of how the old TV show Maverick worked.
Act 1 - Set up characters and situation, and set the catalyst loose. The catalyst is whatever first throws the character's life out of balance and forces him to act. By the end of Act 1, the character has realized this is not a simple problem, and has committed to dealing with the problem.
Act 2 (or Act 2a) - Snyder has two names for this part, and I love them both. "Fun and Games" or "The Promise of the Premise." This is the place where the main idea of the story comes to fruition. It's where all the stuff you anticipate from the movie poster and the trailer happen. This is the part where the mismatched cops are forced to work together. This is the part where the crusading lawyer builds his case. This is the part where the broken down race horse goes into training for the big race.
Mid-point - Act 2 ends when a major event brings reality into that promise of the premise. I had a teacher who called this "The point of no return." The event may be terrible or wonderful, but it changes the dynamic and raises the stakes. This is the moment where the mismatched cops find out they have to get it in gear or the whole city will be wiped out by terrorists. Where the crusading lawyer finds out he'll be disbarred if he doesn't pull this off. Where the owner of the broken down race horse bets the farm on a win.
Act 3 (or Act 2b) is full of reversals. Everything goes wrong, but there are much needed resting points. If the mid-point was a great triumph, then you will immediately have a disaster to shut it down. If the midpoint is a disaster, then you will have a respite in a smaller victory that gives your characters breathing space. Two major things will happen in this slot, although exactly where and how varies by the story. One is that the characters will undergo what Blake Snyder calls "The Long Dark Night of the Soul." Things are at their darkest, the character is out of options, and often characters confess things to each other at this point. Which is related to the other major event of this section: "The Truth Will Be Revealed." This may not be the ultimate truth (as with a mystery) but it will be a revelation that changes everything and allows the character to see what he or she must do.
Act 4 (or Act 3 in Movie-speak) is usually glossed over by movie theorists, perhaps because if you do a good job with the beginning, you'll know what to do here. However, I like to call this part "Bearding the Lion in His Den." This is the part where the protagonist picks up the problem and delivers it back to the source - whether that source is the villain in an action movie, or something more ephemeral in a drama. (For instance, in a movie about alienation between family members, this is the point where the protagonist finally just insists on talking about the thing everybody won't talk about.)
So for me, I think this is a great way to get a handle on an old fashioned serial. Each act has three chapters (making for a 12 chapter novelette). Each act would also have a cliff-hanger. And possibly mini-cliffhangers in each chapter.