So, I went:
1. Set on a space station.
2. Main character is military.
3. More action (but I'm thinking we mix it with a few other genres).
Here's the story I came up with.
Private Jay Avis is a member of a Marine security unit on board a hospital quarantine station orbiting LV-510, a well-developed city colony, which he has called home for about three years. Overall, he's likable, but lonely. He keeps his attention on his job and his unit, who he considers family. He refuses to deal with the doctors and scientists and travelers that come through unless he absolutely has to. It's a very mundane task, checking people's luggage and helping the squad technician run scans of the junky vessels some of the civilians are using to fly to the colony. Once in awhile, a bigger ship--a luxury liner, a supply freighter--will come along and give Avis and his unit more work.
Avis is sitting in the mess hall, which is a very small room lined with plastic crates stuffed with MREs, with the technician--a short little guy with a skinny build and a thick Irish accent, when their sergeant comes in saying a civvie vessel just docked and needs to be locked down immediately. The problem is that one of the female crew on the ship is giving them a hard time. Why? One of the passengers has a thing on his face, and she's begging the Marines to let them in so this guy can get help.
The Marines are all wholly against letting this crew onto the station, so they send their synthetic onto the docked ship to inspect. The android, named Mathias, finds a dead facehugger curled up under a table. The passenger is lying on a bed, and slowly coming around. Baffled, Mathias reports back to the sergeant, who orders the android to contain the facehugger's carcass and bring it onto the station to be tested, and destroyed.
The technician states that the passenger should be put back on the civvie vessel, and the vessel be detached from the station ASAP. "You heard what happened on LV-426, right? If this station is breached, so could LV-510."
The sergeant sends Avis to report this to the station's commissioner. Though the commissioner is reasonable, he explains the station wasn't built to deal with a dangerous, parasitic organism, and gives the Marines permission to do what they can to keep the station safe.
In the meantime, the crew of the civilian vessel accuse the Marines of being overbearing. Obviously, nothing is wrong since the creature fell off and their passenger is fine. While the sergeant struggles with them, Avis returns to his usual post in the docking bay, overlooking the massive airlock and observing people coming and going. Hours pass, and he gets the feeling something is wrong. That's when the alarms start going off. Something's wrong, alright.
His unit corporal runs in, pale and out of breath, ordering Avis to suit up and meet him in their briefing room. Avis trails the corporal to their quarters, and starts putting on his armor with the rest of his squadron. They're informed that there's an Alien on board. They're reminded of what had happened on LV-426.
"We don't want that happening here. Kill it."
The organism is successfully contained in one sector of the station, away from the general population. Of course, the thing's going to try and break out, but not before it slaughters or cocoons the Marines scouting it out.
Some of the Marines do get killed. Avis winds up alone in the bowels of the station, with only a pulse rifle and shoulder lamp. No doubt, he's scared, but is more consumed by rage over his teammates--his brothers and sisters--getting brutally ravaged. Protecting the station now doesn't matter as much as avenging those who had given him a home and family.