The Adventure Seed
The Niflheim system is a cold, barren star system located on the fringes of human-colonized space. With no habitable planets and meager mineral resources, it's only feature of interest is its proximity to a field of dark matter, the mysterious substance which is not exactly physical matter as we know it, and is undetectable by most means, but which nevertheless makes up most of the total mass of the universe. A scientific installation orbits Niflheim-7, a large ice giant on the outer orbit of the system, in order to study the interaction between the dark matter and the planet's gravitational field.
You happen to be visiting the installation when the Ogrix show up; a race of monstrous, methane-breathing aliens which physically resemble large, ugly humanoids. They lay claim to the system and have started constructing some sort of facility on Niflheim-7's frozen surface. They also demand that all humans leave within three planetary rotations.
The Ogrix are a hostile, aggressive race, and their presence so close to humanspace is a definite threat. Also, the Ogrix possess a stardrive superior to anything humans have yet developed. (It utilizes the ultrarelativistic properties of Dark Matter, which is why they are interested in the system; but the humans don't know that yet).
Can you evacuate the scientists from the research base before the Ogrix's deadline? Can you find out what the Ogrix want, and how that might affect Earth? If you can manage to steal an Ogrix stardrive, so much the better!