It has been 300 years since humanity collapsed in the wake of a great war. Nearly all of civilization destroyed, only small groups continue to live on in the wasteland. However, in the wasteland, there are strange towers that have appeared. Seemingly sentient, nobody knows who built them or why, only that anyone who ventures near will know only death. From these towers comes a mysterious substance called diablosis, which corrupts those who touch it, turning them into murderous killing machines. One man, known only as Syd, has made it his mission to destroy these towers and stop the diablosis.
Quantum Theory is a a cover-based third-person shooter similar to the popular Gears of War series. Syd can hide behind and fire from cover, as well as "roadie run" between cover to move closer to enemies. He can carry up to three different weapons at a time, as well as go face-to-face with a powerful melee attack. Syd will also team up with a woman named Filena, with whom he can perform special team attacks, and even throw her at enemies for an especially powerful attack. In addition to the single-player campaign, there are also online team and deathmatch multiplayer modes.