Anyone who is familiar with the Warhammer 40k universe probably already knows this, but adding fluff and subtext to the game makes it all the more fun. In fact, often my group will using in character speech for the marines to give the game a better feel. I thought I'd post some of the assumed endings to the game based on what 4th room you end up with for those not so familiar with the gothic nature of the 41 millennium. First, the original goal of the combat team is the Launch Control Room, but of course that's not always what you end up with. There are many explanations for this. They never had a blueprint or map of the hulk as records were lost and they are blindly searching the hulk using what clues they can find to reach the goal, they are having to maneuver around destroyed rooms or sealed bulkheads, or their combats with the xenos have forced them to use a different route than intended.
Launch Control Room - The original objective. Though the computers are offline, unreliable, and partially destroyed, the marines must make use of them to guide the engines away from the Imperial world or to stop the hulk altogether. Failure means the hulk crashes into the Imperial world and the Genestealers spread through the populace...
Toxin Pumping Station - An emergency measure on every Imperial ship, the Toxin Pumping Station will fill the halls and rooms of the hulk with a deadly nerve gas that will exterminate the xenos. The Space Marines' all-enclosing Terminator Armour will protect them until they can secure escape.
Genestealer Lair - The heart of the enemy, this is the lair containing the Genestealer Brood, including their immature Broodlings and the psychic Hive Mind Brood Leaders that direct the will of the swarm. Eliminating them will turn the xenos into a random, rampaging mob that will tear each other apart. But the Brood Lords are formidable opponents...
Machine Spirit Core - The soul of the hulk and the power source of it's massive engines. Overloading the core will certainly destroy the hulk and eliminate the Genestealer menace from Imperial space. Such bravery and sacrifice will secure the names of the marines in the halls of the Blood Angels forever...
Librarium Vault - Not one of the primary objectives, but before the marines can leave, their Auspexes reveal a hundred or more of the swarm surrounding the room. Taking position, they prepare to sell their lives dearly, unaware that the Emperor is watching and their spilling of xenos blood will cause an Imperial miracle... (This is kind of a wildcard ending, made from best interpretation of the card and goal. Our favorite is that a Sisters of Battle Saint appears and joins the fray to save the marines and help destroy the Genestealers and/or ship.)