Goals are what drive a story, but in a series, there's always a larger goal looming over the entire tale. It's too big to ignore, but also frequently too big to make it the core conflict of the first novel in the series. To keep readers hooked, you have to work your way up to that all-important series goal.
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A series is a complicated thing to plot, because you have to worry about an overall story goal that might span three books to ten books or even more. If the series is situational, like a mystery with a detective, each book will have its own goal and there probably won't be a larger issue hanging over the protag's head. (though there might be a larger character arc goal to work toward as the series develops). But for stories designed to be told over the course of several novels, it takes a lot more work to lead up to that final payoff. Because you essentially have one large story.
Basic rule of thumb says that every book should have a core conflict and that conflict should be resolved by the end of the book. This is the problem you're using to tease the reader into picking up the book.
This is what readers expect to see resolved by the end of the novel, and if you don't do it, they will very likely be mad at you. This goal is the question posed early on in the novel. It's probably whatever your inciting event is, or leads you to. "Defeat the evil bad guy and stop the apocalypse" is a good goal to work toward, but it's vague since any number of things can occur to resolve that goal. Conceptually, your protag spends the entire series trying to do that, but every scene isn't about her "trying to defeat the evil bad guy." What specifically has to happen for that end goal to occur?
What are the steps that need to happen before your protag can do whatever specific thing they need to do to achieve their series goal?
Let's focus on a trilogy, one of the more common same-story series formats. Think of it like a stand alone book broken into acts. In a novel, you have Act One, the setup and inciting event, Act Two, where all the real trouble happens and the stakes escalate, and Act Three, the final showdown with the antag and the climax and wrap up. So your trilogy will have Book One be the setup and inciting event, Book Two will be the middle, and Book Three will be the resolution and climax.
And that's why trilogies are hard to write. Because Book Two is all middle, and we know how tough those can be.
If you look at your series goals the same way you look at your book goals, you can see the major plot points that need to happen over the course of that (three book) story.
Book One = Act One: It's setup, but not in the bad "too much exposition" way. Your protag learns about a problem, something major happens that sets them on the path to the larger story goal (the inciting event). Resolving that event usually resolves the book's core conflict. Just like there are a lot of steps from the inciting event to the plot catalyst (more on that here), the first book of a trilogy will have a lot of steps between the opening scene and the resolution of the book. The difference is that resolving the first book's conflict is also resolving the trilogy's "inciting event" and launches the second act of the story.
Let's use The Shifter as an example.
The Book One, Act One, inciting event is all about Nya discovering her sister is missing. That discovery is what launches the core conflict of the story. By the time she resolves that problem (the end), she's uncovered the fact that the Duke is up to no good. That discovery launches Book Two, same as the discovery that her sister was missing launches the story in Book One. The series goal of defeating the Duke isn't even a blip on the radar yet, but you can see the story is heading in that direction. Nya doesn't know there's a larger series problem until the end of the book, just like she doesn't know there's a book problem at the start of the book. The structure of the first book mirrors the structure of a stand alone novel's Act One.
Book Two = Act Two: In Book Two, you have the middle. The inciting event has created a situation where things get worse and worse and sacrifices must be made. (more on middles here) The Book One goal will be carried over since that's usually what's launching Book Two, but now it's time to throw in the next step of the overall series goal. Just like the mid-point reversal, something will likely happen in Book Two that sends the story in a new direction, often toward what that series goal is.
For example: Nya knows the Duke is bad news by the end of Book One, and her Book Two goal is to prevent him from getting what he wants. She isn't out to defeat him yet, just throw a wrench in his plans. But her actions cause things to get worse, and then she reaches that tipping point where she realizes she's in way over her head and getting out of this mess won't be easy. In stand alone book terms, this is Act Two's dark moment of the soul, that point of no return that happens right before you launch Act Three and head into the climax. For a Book Two, it's often the resolution to the core conflict of that book. Ending on a down note is not uncommon, though you certainly don't have to.
Book Three = Act Three: In Book Three, it's the race to the climax, same as an Act Three. This is where that series goal is most likely going to come center stage. Events have occurred and put the protag on the path where the series goal is also the core conflict of that last novel. (more on endings here) The discovery or realization of that series goal is often Book Three's inciting event. The protag figures out what has to be done (same as they do in a stand alone Act Three) and then takes steps to do it.
For example: Nya finds herself in a situtaion in the beginning of Book Three where she realizes the only way to get what she needs (her book goal) is to defeat the Duke. This realization starts her on the path of the plot of Book Three, and she makes a lot of sacrifices to work her way toward that final showdown with the Duke. (same as your typical stand alone Act Three) She figures out what to do and resolves this conflict. The resolution is satisfying not only for Book Three, but it resolves the larger series goal that was hinted at in Book One, and deepened in Book Two.
If you're plotting a trilogy, try breaking it into the three major turning points of the series goal and conflict.
What is the moment/event where the protag first realizes there's a larger series problem?
This is very likely connected to the resolution and core conflict goal of Book One.
What is the moment/event where the protag first realizes they're completely in over their head and have only made things worse?
This is very likely connected to your Book Two core conflict and goal.
What is the moment/event where your protag realizes the only way to win is to resolve the series goal?
This is very likely the core conflict and goal of Book Three.
Once you've pinpointed those bigger steps, treat each step as a goal that must be resolved per book, and plot from there. Treat it just as if you were plotting a stand alone novel, but instead of leaving things unresolved at the end of an Act (each book), resolve it in a way that would be satisfying to a reader, but still launch the next book in the series. Series goal subplots might be unresolved still, and that's okay. As long as the core conflicts for that book are resolved in some way.
Structure is remarkably the same no matter what part of the book. Scenes follow the same structure as a whole novel, so if you look at the overall series as one big story with parts, it makes it a little easier to see where your goals and conflicts need to be. Same as if you were looking at a scene or act of a single book.