At its most basic plot is the presentation of a problem that needs to be solved: the detective needs to catch the criminal, the lovers need to come together, the world needs to be saved. So when we’re beginning to plot a story we need to think, ‘What is the problem?’ then ‘How are we going to fix that problem?’ and that is your plot.
In The Avengers Assemble the problem is Loki trying to take over the world and The Avengers need to stop him. In Pride and Prejudice a man of good fortune is in want of a wife but Mr Darcy needs to overcome his pride and Lizzie her prejudice. In A Christmas Carol Scrooge is doomed to a terrible afterlife but he can save himself if he overcomes his past. And so on and so on through storytelling.
So when we begin our story we have to present the problem to the reader/audience. Often it might be said that we introduce the characters and then introduce the problem but this isn’t always true. The most common place this isn’t true is in crime fiction where the crime is often, but not always, introduced before the characters, sometimes in the form of a prologue in books or a pre-title sequence in film and television. Sometimes, but not always, this is because the story is more focused on the plot then the characters; for example, in a series where the characters have already been introduced or in anticipation of a series that will give more space to the characters. Another reason can also be because this method throws the reader straight into the story creating a faster pace than a story that builds to the problem appearing. Once again though this isn’t a universal.
This doesn’t mean that the characters are less important than the plot simply that it is the plot that is driving the story forward not the characters. If we were to consider Sherlock Holmes the stories are about watching this unusual man solve crimes in an unusual way, the emphasis is on how Holmes and Watson work together to assemble the pieces. Whereas if we were to look at Jack Reacher the story isn’t about him assembling the pieces through deductive thinking, it’s about him solving the problem before it gets worse and someone or Reacher dies. Usually the more plot driven crime/thriller novels feature a higher body count and/or a time limit and higher stakes then their slower moving counter parts. Even in Hound of the Baskervilles at no point do we seriously think Holmes or Watson are in danger of being next on the killer’s list.
Whether we introduce the characters first or second for there to be a plot there has to be a primary problem (we’ll get to subplots). If we can’t say what our character’s main problem is and their goal (the solution) then we might need to either go back to the drawing board or do some serious editing. The problem we have once we’ve given our characters a problem is, how do we solve it?