Nemesis Gate is set at the end of the Fusion Age (PL 6), in AD 2201. Humans have explored and colonised much of the Solar System. In the process, we discovered the Nemesis Gate, an alien artifact beyond the orbit of Neptune which allows travel to other solar systems hundreds of light years apart. Using the Gate, humans have colonised five other solar systems and made contact with three other intelligent species – the t'sa, weren and sesheyans.

In this setting, space is big, dark and full of terrifying mysteries. Several of the known solar systems have natural hazards or dangerous alien artifacts which can annihilate careless explorers. The alien species are mysterious outsiders with their own secret agendas. The greatest danger comes from the Gates themselves: a terrifying unknown force called the Darkness which can bring a hapless spacefarer's worst nightmares to life.

The human sphere is dominated by two groups of political powers. The territorial powers of Old Earth are the New UN, the United State of Holy America (USH) and the Non-Integrated States. Most of Old Sol is owned by four mega-corporations, each of very different nature: Virgin Space Enterprises, Coca-Cola Lockheed, Sony Ford Microsoft, and the Muhammadiyah. (See separate briefings for Sol, Earth, the known systems, and the corporations.)

The t'sa have a more advanced technology than humans, though their society seems backward and naïve. The UN and the Fifth Line of t'sa are joint partners in developing the Ketann system. The t'sa hold an unknown number of solar systems beyond Ketann. (See species briefing: t'sa.)

The weren had advanced to the industrial age (PL 4) before contact with humans from Shell Rothmans Corporation. Many weren have been indentured to Coca-Cola, who own their home system. They work as labourers and security forces, both in the Oasis system and Old Sol. (See species briefing: weren.)

The sesheyans are highly mysterious. Though their culture and technology is Stone Age, they are found in two solar systems of human space and at least one t'sa system. Numerous alien artifacts are found on their worlds, in the Tau Ceti system and Ketann. (See species briefing: sesheyan.)

The Gates themselves are known to be hundreds of millions of years old. No human can imagine the technology which would be needed to build one. The only other artifact of the Gate Builders is the Tau Ceti Object.

Artifacts of the race, or races called Forerunners are much more numerous, down to the size of personal equipment. Some theorists count the sesheyans themselves as evidence of the Forerunners. The most spectacular Forerunner artifact is the Tower of Angels in the Eden system, an enormous automated battle-station which attacks any ship emerging from the Eden Gate.

Rumours of new alien species are common, but no evidence is available to the general public.