In The Plutonia Experiment, after Hell's catastrophic invasion of Earth, the global governments decide to take measures against any possible future invasion, knowing that the powers of Hell still remained strong. The UAC is refounded under completely new management (the old trustees and stockholders were all dead anyway) and aims at developing tools that would prevent demonic invasions. The scientists start working on a device known as the Quantum Accelerator that is intended to close invasion Gates and stop possible invasions. The experiments are carried out in a secret research complex, with a stationed detachment of marines. The work seems to be going well but the creatures from Outside have their attention drawn towards the new research. A Gate opens in the heart of the complex and unnatural horrors pour out. The Quantum Accelerator performs superbly: the Gate is quickly closed and the invasion is stopped. Research continues more boldly. On the next day, a ring of 7 Gates opens and an even greater invasion begins. For one hour the Quantum Accelerators manage to close 6 of the Gates, but the Hellish army has become too numerous and too strong. The complex is overrun. Everyone is slain, or zombified. The last Gate of Hell remains open, manned and guarded by a Gatekeeper: a powerful, enormous and ancient demon that has the power to open Dimensional Gates and control or protect them. The government, frantic that the Quantum Accelerator will be destroyed or used against humanity, orders all marines to the site at once. The player, the nameless space marine, was on leave at the beach. He was also closest to the site and gets there first. There he discovers that there is much demonic activity (howling, chanting, machine sounds) within the complex; the Gatekeeper is obviously working on something, and his work would soon reach some awful climax. He also realizes that when the marines arrive, they would not be able to penetrate the heavily infested complex, despite the firepower and support they will have. The marine decides to enter the complex and stop the Gatekeeper alone.