The "I have a question" or "Something's happened, how do we fix it?" plot structure. As previously stated, this structure is practically form for the mystery and horror genres, but other genres can use it too. Here's the basic rundown of the Idea plot.
Something has happened that can't be explained. It could be that someone has died or gone missing or that an alien spacecraft has fallen into someone's backyard. It doesn't really matter what it is so long as there is a question involved and that question is always: Why? Why is that person dead? Why is this person missing? Why did an alien spacecraft land in my backyard? The point of the novel is to answer that question and the many subsequent questions that will follow.
So here is our practice plot:
A family of five (Mom, Dad, Daughter, Son, Daughter) live on a farm that is slowly falling into ruin because they can't afford the upkeep. They've sold most of their animals and only kept their one milk cow and two chickens. A recent storm has blown most of the shingles off the roof and the house now leaks so Dad and Son spend most of their days trying to fix it. Mom and Daughter 1 spend most of their time making quilts and other crafts to sell at the local market, so Daughter 2 is often left to her own devices. For convenience sake, Son will be named Thomas (13), Daughter 1 is Amanda (16) and Daughter 2 is Mary (5).
After a long day's work, the family comes together for family dinner only to realize that Mary is missing. Mom vaguely remembers the girl saying she wanted to go play in the woods and has a horrified moment when she remembers that, due to the recent heavy rains, the nearby creek has swollen enough to become a small, but deep, river. The parents and children quickly split up to try and find Mary and bring her home.
So now that we have our stage set, where is the question? Here:
Why is Mary missing?
There are several different possibilities as to why Mary is missing and those possibilities bring up sub-questions such as:
Did she fall into the river? Did she get kidnapped? Was she injured by falling out of a tree? Or did she simply lose track of time?
Now let's answer a few of these questions.
Thomas makes his way through the woods to his sister's favorite spot. He finds the ring of rocks she made (which she likes to pretend is a fairy ring) and next to it is her plastic fairy wand. Taking a look around, Thomas notices a ring of mushrooms that he'd never seen before. Knowing that Mary would have been drawn to it, Thomas goes to the ring to search for clues. He inadvertently steps inside and is confronted by a real-live fairy.
So, why is Mary missing? Because she was kidnapped by fairies. We've answered one 'Why' question, but there are others that need to be addressed such as:
Why did the fairies kidnap her?
Where is she?
How can Thomas get her back?
The fairies kidnapped her because they plan to replace her with a changeling, so they took her back to their Fairy Garden, which exists in a magical time-dilation where 1 week equals 1 day in the real world. After solving a fairy riddle, Thomas is permitted to enter the Fairy Garden where he is told he'll have one week to pass a group of trials if he wishes to recover his sister and return home. If he fails, then he and Mary are stuck in the Fairy Garden forever.
This, of course, brings us to a new set of questions:
What sort of trials are they?
Can Thomas pass them?
Where is Mary being kept?
If he does fail, is there still a way to escape?
The answers to these questions is where you'll find your conflict, motivation and, eventually, your resolution. Question upon question will continue building and building until, finally, you've reached the final question which is:
Now what?
If you've reached the 'Now what?' point, then congratulations, because the answer is almost always:
The End.
Happy Writing!