The Scrin have actually been in our solar system for many centuries, concealed behind an asteroid beyond the orbit of Neptune, their ships dormant and their crews in suspended animation. They waited quietly as the human race fought its way out of the dark ages and climbed the ladder of civilization. They waited as the first radio waves were broadcast into space, the first nuclear weapon was used in anger, and the first primitive human spacecraft were launched into low Earth orbit. They waited for a signal, a blast of radiation that finally arrives just as the intensity of the Third Tiberium War on Earth reaches its peak. When the signal is confirmed, the onboard artificial intelligence systems begin bringing the huge alien vessels to life, waking up the crews, and setting a course for Earth.
When the Scrin arrive on Earth, they land in the Tiberium-infested wastelands designated as Red Zones by GDI. Vast Scrin armies pour out of the Red Zones and engage GDI and Nod indiscriminately with seemingly overwhelming force. At first, there doesn't seem to be a pattern to the alien assault. Later, GDI realizes that the Scrin aren't really interested in the human race. They're here because of the Tiberium on Earth. The game story will unravel some of the mystery surrounding the arrival of these aliens and their relationship with the green crystal. Many questions will be answered, and others will almost certainly be raised--the mythology surrounding Tiberium and the Scrin runs very deep.