Hey Grant, here some ideas or your fan fiction.
1. You can make it where these 3 astronaunts are in space.
2. Then here's this space demon that attacks the ship.
3. Nasa call the gb's out help them.
4. Once they settle through the space training, they're out of orbit, they arrived to the ship.
5. They enter the ship and they figure out that there's a gas leak and its causing them to have halluciantions.
6. But they later figure it out so they repair the gaee leak and they go pursuit for the demon.
7. they capture it, save the astronauts, and return home.
Those Ideas should help you out.