The contents of a synopsis should cover the major plot points of your novel. Publishers ask for a synopsis to see whether you know how to plot a novel before they read it. So a good place to start is the theory that there are only seven basic plots and identify which one(s) your story uses. That basic plot is the template for what information to put in your synopsis. Highlight both where you follow the basic plot and your personal twists.
Here are two example templates I’ve created based on two of the seven plots.
Synopsis of Voyage and Return by Archetypal Plot
Hero is restricted somehow but open to a shattering new experience. He is young and naïve, and curious and looking for wonder, and perhaps bored, drowsy, or restless. Hero is thrust out of his familiar, limited world into a strange world unlike anything he’s known before.
He explores his new world, fascinated by its puzzles and unfamiliarity. Hero’s restrictions are lifting. However, he never feels truly at home here.
Gradually, his difficulties and frustrations increase, drawing bands of a new restriction around the hero. A Dark Shadow grows and becomes increasingly alarming.
Dark Shadow dominates more and more, and begins to seriously threaten Hero’s survival. Hero is now even more restricted here than he had been at home.
Danger closes in on Hero and his suffering becomes unbearable. Just when all hope seems lost, Hero escapes from death at Dark Shadow’s hands by returning to the world he came from. Hero loses the positive relationships he’d gained in the other world, but Hero has learned from his experience and grown in character, so his life has been forever changed positively.
Synopsis of Rags to Riches by Archetypal Plot
Heroine is young and living in wretched conditions at home. She is lowly and unhappy. A dark shadow looms of malevolent rivals who scorn and maltreat her. A helper arrives (or an event occurs) that calls her (sends) her out into a wider world.
New ordeals crop up. Heroine enjoys a first, limited success. Along with her improved fortunes, she gains a relationship that becomes her Precious and bests her Dark Rival(s) for now. She is not yet mature enough for her final state of glory.
Everything goes wrong. Dark Rival’s shadow returns. Heroine loses all she’s gained and is separated from her Precious. She is overwhelmed with despair in her darkest hour.
Heroine emerges from this darkness into a new light. She is still not everything she could be, but is discovering in herself a new strength. Once she matures into this, her strength is put to the final test as she must confront the dark rival, who stands in between her and her goal. At last, she successfully resolves this conflict and the dark shadow is removed from her life entirely.
Finally liberated, Heroine is reunited with her Precious. She enters fully into her own as she is now ready to receive God’s full purpose for her life and use what he gives her wisely.
If you like this technique, you can read about all seven basic plots in: Seven Basic Plots: Why We Tell Stories by Christopher Booker.