Here's mine: We start off with the characters. They've all been given orders from some very anonymous Brass, that nobody can point a finger to,(That detail has to be added in, because SOMEONE'S going to ask about it,) go to Venus, on a base that, as far as they know, doesn't exist. To do something that sounds like total BS...They're there to train new recruits. Except that, as everyone knows, that particular section of Venus is very much a warzone. You don't train new recruits in the middle of a war. That's how it all begins. That's the little backstory you have as to where you are. Then comes where you actually are: You're sitting in a conference room, attending a meeting of a sort that nobody on the ship seems to understand, nor know anything about. All around you are the other chaps assigned to this...Training mission. Your commander then gives you the statement you've all been waiting to hear: Your official orders were BS. What you were really here for was a little recruitment, and the voyage a little token by the group you're getting recruited into: The Interstellar Marines, or the IM. He steps out of the way, then, and the projector comes alive with a chap who's very obviously Brass, and he gives you some generic reason on how you're the best they could find, yada yada yada. Continues on, finally gets done BSing you, and gives you the real debriefing. You're going to that particular area of Venus because they have reason to believe that extensive genetic and technological testing's being done there, and they need a group of people skilled enough to get the job done, and hopefully bring back some samples. Perhaps even test them, while they're at it.
So, they go in, have a few small encounters with the local military, and obviously win. Suddenly, human soldiers give away to very much tiger soldiers. With the human maturity level of twelve-year-olds with guns. As the fighting continues, you think you've gotten through the mission, you grab the samples, if you do a few optionals, you get a shiny new gun or helmet or some such, that's come up with on the spot, you head out. Well, transmissions have been cut, for no apparent reason, and the evac ain't comin'. Nope, they haven't abandoned you; while you were down on-base, a cloaked Satallite system was engaged as part of the security. Not only did they cut communications, they turned your transport into a nice fireball. The base was what one might call a Venus fly-trap. And you're the fly.
So, now you go onto your next mission: Escaping. You're thrown into the wilderness, where you have to fight, guess what, the somewhat more dangerous genetic experiments, let loose to guard the area from anyone that might get to curious. In this case, giant centipedes, bears, and a super-mutated moose. Along the way, you find bits and pieces of what seem to be standard military armor. Continue on, continue on, and finally, you reach the end of this particular segment: A transport, intact, absolutely covered in blood. All around it are the remains of armor and marines. The chaps who stumbled upon the base. Upon entering the transport, you may note that it's got enough fuel to get you out of the area, and it's about 80% functional, with flight capability.
Of course, you can't leave just yet, because now you've got to take down the satallites that are keeping any transports from getting in or out of Venus. Fortunately for you, along the way, the GM probably bumped a seemingly insignificant piece of technology or information into your group, that now comes in handy. You have the rough coordinates of a base. Deciding that, hey, what the hell can go wrong? You decide to head there. Upon reaching the base, your transport gives. You end up crashing into the base. You also end up having to either fight or get captured. Actually, it's just getting captured. This is an important plot-point, as while captured, you get to overhear some very important information pieces that'll allow you to figure out there's a little more to this then meets the eye. Namely, that the base from before was just there to trigger a large-scale offensive, that would take Venus in one fell swoop. So, of course, after some typical player-innovation(Or a plot device, if you've got some weak roleplayers,) you break out, find the control area, deactivate the satallites. It is, unfortunately, too late. There will be no rescue-team. No force there just in the nick of time. Instead, the military decides to bombard the area. All troops are ordered to evacuate, and, of course, there are no civilians in that particular portion, so it's safe.
That's the first part of the plot.