The year is 2201. In six months, Earth will be gone – its last inhabitants clinging to life and hope aboard the ISCS Armstrong.
But they are not the only humans left.
Over the last year, the Hunters have systematically destroyed 12 of the 18 colony ships in the galaxy. Each time, they arrive and attack with so little warning that there is no time for the humans to do anything but be slaughtered.
When the Hunter fleet approached the Theta Sculptoris system in December of 2201, the ISCS Pérouse was performing a deepscan of a nearby gas giant. As a result, the human crew detected the thousands of Hunter ships on an intercept course almost by chance.
With an extra day to prepare for the inevitable, the Pérouse did the only thing it could – alert the nearest colony ship to the imminent danger and hope that someone else would have a fighting chance. The ship's scientists successfully converted all the onboard computing power into a focused microwave burst transmission to send a distress call.
Sending the transmission burned out all of the systems aboard the Pérouse, and it was unable to mount any resistance to the Hunters. However, the recipients of the burst transmission would meet a far different fate.
The ISCS Tachibana was already special when it launched from Earth in 2156. As Japan had already launched the world's first colonization vessel (the Chrysanthemum), its second effort was more experimental in nature. While it still houses all the traditional locales of a colony ship – Hydroponics, Maintenance, Colonist Quarters, and so on – it also features a more established military presence. And while the other colony ships had military personnel aboard, the Tachibana is unique in that a significant portion of its population is present specifically because of their service.
With the Tachibana, the ISCI had hoped to study the feasibility of permanent, space-based installations of intellectual, industrial, and military operations. As such, there is a post-graduate school and research center in the Colonist zone, an experimental comet dust-based power plant in Engineering, and most significantly, a full complement of twenty battalions of ground- and space-based combat forces.
It is this combination of disciplines that would save the lives of everyone on board the Tachibana. When the crew receives the focused burst transmission from the Pérouse, the shock and panic in its wake is swiftly quelled by the level-headedness of the established forces.
The Tachibana's officers agree with the findings of the research center: the only possible way to survive the Hunters' juggernaut (which will arrive in less than three days) is to create a multispectrum volumetric torrent. Essentially a hologram that can fool any method of scan, the MVT requires a massive amount of energy and processing power – enough to completely fry the Tachibana's engines, and irreversibly sever its connection with the ISCI network.
However, even though the Tachibana is totally cut off from the rest of the galaxy, it is also safe from its predator. When the Hunters arrive in the Zeta Reticuli system to continue their campaign of annihilation, the Tachibana's MVT tricks their sensors into seeing a destroyed colony ship with zero life signs aboard. The main fleet quickly moves on, leaving one medium-sized attack craft behind to clean up the mess.
In the single-player world of Space Siege, Seth Walker is the only one on the Armstrong who fights enemies. But in multiplayer, onboard the Tachibana, there are thousands of potential Seths – like robotics specialists who attend the ship's postgraduate facilities, or military veterans who have spent their lives studying weapons. And it's a good thing they're around, because life on board the Tachibana quickly becomes just as dangerous as it is aboard the Armstrong.
Even though the bulk of the Hunters' fleet left the system soon after their scan came up empty, the remaining attack craft arrives fairly soon after. Once it is close enough to weapons range, the Tachibana engages a concentrated tractor beam and pulls the Hunter ship close. The two vessels collide, and the Hunters' communication with the rest of its fleet is quickly disabled.
Knowing what has happened to their fellow humans, the crew on board the Tachibana is eager to enact some vengeance on the Hunters. The fighting between the two forces will be frequent and fierce. But even after the two sides board each others' ships and the fight becomes omnipresent, the humans and Hunters will face yet another foe.
When the Tachibana engaged its MVT, its crew accepted the irrevocable damage to the engines and network as part of the cost. However, an unforeseen side effect of the MVT was that the Tachibana's individual subsystem AI's are hopelessly corrupted. While this is not immediately apparent to the crew, it soon becomes rather obvious, when AI-created cyborgs and corrupted robots begin attacking the humans.
While PILOT's rationale for its actions stems from a desire to preserve humanity at all costs, the Tachibana's subsystems (who call themselves "The Family") are like computers infected by a virus. There is seemingly no rhyme or reason to what they do – and because of this, the humans aboard have a great deal of difficulty fighting on two fronts at once.
It is here where the multiplayer component of Space Siege begins: robots, aliens, and cyborgs are everywhere, fighting the humans and fighting each other. The only safe place is the ADF's barracks on board the Tachibana, where the thousands of combat-ready crew members can form up strike teams and fight for survival against the corrupted Family and desperate Hunters.