Quantum Theory is a third-person, cover-based shooting game developed by Team Tachyon and published by Tecmo Koei. Since its reveal, the game was noted to be "heavily influenced" by the Gears of War series, inasmuch that every single gameplay mechanic was lifted directly from the Epic Games franchise. The game is set after a great world war has ravaged the land and killed off the majority of mankind, leaving only a handful of people alive. These people have developed their own community, known as Cocoon, a name stolen from Final Fantasy XIII, which has recently become under threat by a mysterious dark entity known as "Erosion". Erosion is spreading across the land, threatening to wipe out Cocoon and all who live there, and so the humans recruit the talents of various mercenaries, who set out to fight back against Erosion, by destroying its source, found within the "Living Tower", this is where most of the game takes place.
Players take on the role of Syd, who, along with his companion Filena, must fight their way to the very top of the tower and discover the secrets it hides. The fact that the tower is essentially 'alive' makes for an interesting gameplay mechanic as much of the cover within the game can be prone to changing shape, appearing out of the ground, or disappearing altogether, hence the player must always be on their toes as no one piece of cover is safe.
The game begins with Syd and a girl named Nyx attempting to escape the crumbling Ark; Nyx, somehow being tied to the structure, dies as the tower does, but Syd manages to escape. Syd then travels to another Ark, meeting Filena in a stand-off. Syd is a Gillskin, while Filena is a Nosferatu, a pair of non-human warring factions. It's revealed that Syd is trying to destroy the tower, while Filena is trying to purge it from the "Erosion," a black substance that infects the environment and creatures with "Diablosis".
They separate and meet up again several times as they ascend the tower. Eventually Syd discovers that Filena is a humanoid created by her father. If she is engulfed by Diablosis, it will allow the disease to spread even further. Of course, Filena is taken by the Diablosis and freed by Syd.
They reach the top of the tower and meet the "brain" of the tower, which instructs Filena to kill Syd. She considers the offer, but destroys the brain instead, leading to a sequence that mirrors Syd and Nyx's escape from the tower at the beginning of the game. The fate of both Syd and Filena is revealed in a post-credits scene in which they take on another Ark together.
As befits a clone of Gears of War, players must take and use cover effectively in order to wipe out waves of enemies and progress to the next area. Syd is able to sprint and stick to almost any part of the environment and use it as cover, from here Syd has the ability to blindfire or quickly mount over or run around whatever cover he is using. Shooting is either done from the hip, or aiming down the sights which provides increased levels of accuracy.
What makes the game unique however, is the presence of Filena, at any time, as long as she is nearby, Syd can pick her up and hurl her at enemies which she will then attack, dealing quite a large amount of damage and, in some cases, an instant-kill. Filena also can perform certain combo moves with Syd after he uses a melee attack on an enemy. There are also some brief 'platforming' sections to be found within the game when the tower begins to rapidly shift and various platforms will move or change shape and Syd must make his way across them in order to progress.
In terms of multiplayer, the game offers online Team Battle and Battle Royale modes, which are essentially Team Deathmatch and Deathmatch across a variety of maps pulled from the campaign.
Quantum Theory received generally negative reviews. GameTrailers called it "flawed, unoriginal, and dated". Eurogamer lambasted it for its "uninspiring, repetitive internal environments" and "nothing short of a direct copy" of Gears of War (a common complaint among reviewers).
The game has also failed to hold the interest of the multiplayer crowd, with lobbies reported to be completely empty within days of the game's launch. It's virtually impossible to get a game going with others, rendering the mode inaccessible.