I am not a writer so have no idea on screen play writing. However I do love a good Sci-Fi film anything from Aliens, Star Wars, Independence Day even Doctor Who. I love them all.
To help me think of my story line I would have to think of certain aspects of the film -
- The main character - who are they?, why they are there?, how can the audience relate to them?
- The setting - being Sci-Fi the environment holds no boundaries.
- The event - what is going to happen to our main character.
- The outcome - how does it end?
CHARACTER
My story would be based around a male scientist who has travelled to 
the moon investigating whether life is sustainable there. He would be from a happy home and gone through normal schooling, a side from being a bit of a loner he would be happy with his life, as science is his life. He took this opportunity to research the moon as a great way to progress his career.
Initially you will not see much of him, it would be shot from a 1st person point of view as he is an Alien and slowly the audience would be introduced to this at key points within the story. The more the audience knows about his alien nature the more you see of him. Slowly he would also suffer from flashbacks, initially they would be just noisy and blurry.
SETTING
It is based on the moon so all our gravity laws apply and he would have to wear a space suit the same as humans. The aliens are not much different from humans when it comes to basic survival, they require oxygen to breath, they have basic organs within their bodies, the are not super intelligent, however they have heightened senses and fast reflexes.
They have set up camp and are working hard to investigate the area and doing various research, when......
EVENT
A group of humans come to the moon and start attacking them, they 
are looking for missing spacemen who had not returned. Presuming the Aliens had killed all the humans they keep on bringing wave after wave of war fare.
Understandably gun fire would not work in space so it would be lots of hand to hand combat. The camp that the Aliens have set up has artificial gravity so spares, crossbows and guns are able to work inside but they have to be careful where they shoot.
This brings the group together and makes them retreat back into a derelict part of the camp. At this point the intensity of our main characters flash backs increase you realise that they are memories of humans.
As the fighting continues the group gets smaller and they get pushed more and more into this abandoned area where they start to discover all the human work and ultimately they find out that in fact they were humans (they were his memories of a previous life) and that what they found on the moon converted them to this Alien species.
OUTCOME
The Humans do not listen to our Aliens when they try to explain the danger they are putting themselves in (it is hard in a different language) and they keep on advancing (slowly changing) until the have the final battle in this one room where the remaining humans turn into the Aliens in a really gross way.
The End
I love films with lots of action so my film would be full of it, plus a good character I can cheer for. So hopefully this story would encompass that.