The plot is basically the structure of your book. The skeleton which maintains your story.
The basic requirement is that there is a problem or conflict. Without conflict you are merely telling an anecdote.
The the other plot requirements are:
A beginning with a 'hook' to grab the reader, which introduces characters and setting and gives some hint of the conflict or problem.
A middle in which the conflict is developed and there is some attempt to resolve it, probably not successfully at this point. The pace should vary, with fast tense bits and other slower paced situations. The 'rule of three' is often a good way of structuring, with two unsuccessful attempts to sort the problem followed by a final successful resolution. This is a classic structure, obvious in many traditional children's tales. For example, in the Three Little Pigs, the pigs make houses of straw and wood before they take refuge in the brick built house.
The end should have the final resolution of the problem. This does not have to be a 'happily ever after, clichéd ending, but it does have to be satisfactory to the readers. Yes, I know you cannot please all the people all the time, but you have to make sure you've played fair and done your best. Disappointed readers won't come back for more.
How do you plot your novel?
That is up to you, the writer. Some people start with an opening scenario and starting characters and play with the action. This can be great fun but sometimes it paralyses the writer when they end up thinking, 'what do I do now?' or 'Help, this novel is totally out of control. Just letting it flow can make the novel incredibly fresh and exciting, but it does involve the willingness to not just do a quick edit but to rewrite meticulously and occasionally go back to near the beginning and take a completely different path. If you write crime, as I do, it's a great way of fooling the reader about whodunnit or what's going to happen next. The important thing is that when you've completed and are ready to publish the work, you have it all in control.
Many people plan their novel, meticulously; often taking nearly as long to sort out the plot as they take to write the novel. That can be great, as long as you don't turn the novel into a 'write by numbers' trudge. Never be afraid to explore the wonderful diversions that the writing process reveals and that your characters suggest.
One vital rule: as with characters that serve no purpose, if a scene in a story serves no useful purpose to further the story, it has to go, otherwise it will bog down the story, whether it's a 100,000 word novel or a 1,000 word short story.
Of course, if writing a novel, you can have sub-plots to enrich the action but in a short story the important thing is to keep it simple and stick with a single plot-line.
I love the word clue, which in its Old English form is clew, and means a thread. Whatever you are writing you need that thread of plot, place and people, which, like Theseus in the labyrinth, will lead you safely through.
In a workshop last week, the students grew very excited when fitting books they had read and their own work into the scholarly guidelines of the seven basic plots:
Overcoming the Monster; Rags to Riches; The Quest; Voyage and Return; Comedy; Tragedy; Rebirth.
Give it a try. It's interesting to work out the plot that fits your book and may help to clarify your structure.
The theme is part of plotting, but it can be argued that it's also part of characterisation and setting. It's the essence of your story; it is the deeper meaning within it. The theme can be defined in a few words, which will have some relevance to the problem your central character has been dealing with; for example: survival; loss; regret, self-discovery.