Killzone
In the distant future, humanity has colonized space. Altercations between humans causes them to split into two distinct factions. The ISA remain on Earth, but their enemies, called the Helghast, have been exiled to another planet. On that planet, they have changed forms and become something other than human. Eventually, the Helghast launch a war against the ISA, determined to reclaim the Earth from their enemies.
Area 51
Something has gone awry at the government's secret Area 51 facility. Ethan Cole, a bio-hazard operative, is dispatched along with his team to carry out an investigation. He soon finds out that a lethal virus has been released and the facility is full of vicious aliens. As if this wasn't enough of a problem, there are even more conspiracies to be uncovered along the way.
Red Faction
A Martian mining company, the Ultor Corporation, draws many people from Earth to come work at its operation. Parker is one such person, but he soon discovers the rampant abuse at the corporation. Ultor employees are forced to work extra-long shifts and are subjected to pitiful living conditions.
Additionally, a strange plague seems to be floating around that is killing miners instantly. When Ultor refuses to get involved, Parker and the miners launch a rebellion against the corporation, and attempt to right the wrongs they've dealt with for far too long.
Half-Life
Gordon Freeman is a new anomalous materials employee at the Black Mesa research facility. On his first day, an experiment goes horribly wrong and tears open a rift between dimensions. Through the rift comes a race of evil aliens, determined to make the Earth their new home. Soon, Gordon realizes that most of the employees have been killed by the aliens, and he's largely on his own. Before long, a military task force shows up to destroy not only the aliens, but also anyone who knows about the invasion.
TimeSplitters: Future Perfect
The TimeSplitters are a vicious race of creatures with the ability to travel through time and destroy history. The time-traveling soldier, Cortez, must make his way through past and future, dealing with the invasion from 1914 to 2401. Along the way, Cortez must figure out how to use the time paradoxes he's creating to his benefit and keep them from causing even more trouble.