SIMPLE PLOT LINES
Now that you have your cast of characters, you need to come up with more plot details and decide on what story arcs (major plots and subplots) you'll need to answer your Big Question.
An arc or plot line is a simple statement of plot. You can create one for the main plot, the secondary plot line, and for the subplots.
For STAR-CROSSED, the science fiction story arc is
Mara must rescue Tristan from the harem, and they must escape to freedom.
Since this novel is a romance as well as science fiction adventure, there is also a romantic story arc:
Mara and Tristan must put aside their cultural differences to find love. Tristan must choose between Mara and Dorian by giving up his own insecurities about love and by discovering whom he loves.
Here are story arcs you'll find in different types of romance novels.
In a romantic suspense
the romance story arc--
the hero and heroine's emotional story arc
the mystery/suspense arc--
the villain's story arc
In category romance (series lines like Harlequin American)
The romance is the major and usually the only story arc
Sometimes, a minor subplot may cause conflict between the hero and heroine
Fantasy, science fiction, and straight mystery also have one major story arc. Fantasy and science fiction tend toward the quest or task story arc. Romance and character relationships are more often a subplot arc or character development with no major plot importance.
Now you need to detail your own story arcs and plot elements.
Get a sheet of paper or a blank page on your computer. First jot down the ideas you've already come up with thanks to creating your question and the characters you've created.
Now start writing down the obvious plot points.
For STAR-CROSSED, the most obvious ones involved the story arc Mara must follow to fight the corrupt government, to save Tristan and Kellen, and to destroy the government.
Each attempt she makes is thwarted in the early stages, and each time she's thwarted, the situation becomes more hopeless.
Since this is a romance, I also had the romance story arc.
Here are some other notes I made on the novel.
Mara & Tristan in unfeasible power positions, a struggle to regain equality between them.
Cadaran as embodiment of the evil government and the evils of the harem. Tristan's best friend Kellen must become Cadaran's bed slave and faces the true indignities of the harem which Mara spares Tristan from.
Kellen vs. Cadaran, Kellen's attempts at escape -- major subplot.
Another plot conflict/subplot: Tristan's female friend Dorian must discover that Tristan & Kellen aren't dead, and she figures out about the harem planet and must come to their rescue.
Emotional conflict from this: Dorian believes herself in love with Tristan. Tristan uncertain of his feelings for her. Dorian's presence will tear apart the fragile bond between Tristan & Mara as his escape releases him from Mara's control.
Tristan's emotional conflict-- anger at harem society with Mara as representative vs. love for Mara as individual.
Theme: freedom through love, the importance of trust.
Kellen as foil to Tristan
Kellen's emotional conflict-- hatred of society and struggle to retain emotional dignity.
his inner freedom vs. the hopelessness at being victim of an inescapable system.
Possible small conflicts:
Mara's housekeeper Novia acts as spy for Cadaran.
Mara's intelligent alien pet Floppy hates Tristan.
Mara becomes laughingstock when she takes a bed slave because her beloved dead mother was opposed to sexual slavery.
Notice that I also gave suggestions for different themes within the novel that are a bit different from the original question, but reflect the original question. Doing this will add even more depth to your novel.