When the Abyss Men – the third generation of subatlantic colonists – return to the surface, starting a war with the native tribes, a reckless young woman from one of the most violent tribes is ready to do whatever it takes to keep her little sister safe – both from the Abyss Men and her own tribe.

One challenge with loglining ?SciFi stories is that they are about unfamiliar worlds. ?Since the brevity requirement precludes explaining that world, it is usually better to describe the story in familiar terminology. ?Introducing unfamiliar terms in a logline is usually a stumbling block.

So while subterranean is a familiar term, "subatlantic" is not. ?Now, ?I can guess "subatlantic" is akin to subterranean, but I shouldn't have to guess. ?The logline is a promotional tool, to get movie makers to read the script. ?It should entice -- not puzzle.

fwiw

Patricia,
First off, I come to this site to help others develop their loglines, and in doing so I believe I have a much better grasp on stories and how to write loglines. So I recommend reviewing other people's loglines, studying them and trying to identify what works and what doesn't. Try going to the Examples tab and looking through loglines of released films and tv shows, maybe even read the scripts or watch them and decide for yourself if the logline fits. I for one haven't posted many loglines, but any feedback on my latest one is welcome.

Anyway, as DPG said that is quite a bit of background. And it's obvious that you've spent a great deal of time on it. But film is a visual medium, so if you choose to keep your story the direction it's going, then don't tell the audience that background. Just show it, give little glimpses. Only give a few lines of dialogue to explain some things. That brings to mind an excellent film: Children of Men. It doesn't just tell the audience everything, the world feels lived in and has a history that the audience slowly fills in themselves. There's an excellent Nerdwriter video about it. (even though Children of Men doesn't have nearly that much background and history to it.)
As for the logline, in short, what is her goal? Ask yourself, at the end of the climax, what has she achieved or failed at? And how does she go about pursuing that goal? Is the goal to stop the war, protect her sister? Focus the logline, and the script, on one goal.
I hope this helps.

Have you read H.G. Wells "The Time Machine"? ?A sci-fi classic. ?He ?elegantly ?and efficiently establishes the back story of how homo sapiens split into 2 species, the underworld species, the Morlocks and the surface world species, the Eloi. And he has a thematic purpose in splitting the purpose.

What's te thematic purpose in the split in your story? ?What's the ?(subtext) conflict behind the (text) conflict? Or is it conflict just for the sake of conflict?