STORY PT. 2 (SPOILERS)
Almost immediately following this, the cult/terrorist organization called the Church of Unitology, attacks the colony. The unitologists intend to kill Isaac because he possesses the unique ability to destroy the “markers”, alien artifacts which the unitologists believe are divine. The markers however, also happen to be the things which create necromorphs. Isaac, now having very little choice in his course of action, follows the soldiers. After escaping capture by the Unitologists, and surviving a new necromorph outbreak soon after, Isaac eventually escapes the colony and travels to a planet called Tau Volantis, where Ellie is believed to have sent out a distress signal from.
Tau Volantis and it's accompanying orbital junk yard
The orbit around Tau Volantis is littered with debris and abandoned ships from a 200 year old expedition to the planet. It is in one of these ships that Ellie’s distress signal is originating. As Isaac’s ship approaches, he passes through a mine field which destroys the ship and strands Isaac and the other crewmembers of his ship in the wreckage Ellie has been broadcasting from. Once reunited with Isaac, she explains the reason he has been brought to this place: The markers have been around for a long time, and the centuries old expedition had discovered the source, or homeworld, of the markers. They had even discovered a way to "turn the markers off", something Ellie is convinced will stop the necromorphs for good.
Unfortunately, the expedition fell victim to the necromorphs before the markers could be turned off, and so Ellie had set out to finish what the expedition had started. Isaac was in a unique position to help, having survived previous necromorph encounters and having been given an imprint of the markers’ blueprints, or “DNA” in his head, during Dead Space 2. This makes him the only person able to destroy the markers, and thus he is invaluable to her task. But between him and their goal is an army of mutated corpses with knives for hands. And a radical religious cult. And the ravages of deep space. Oh boy.