Halo: Combat Evolved is a 2001 first-person shooter video game developed by Bungie and published by Microsoft Game Studios for the Xbox, for which it was released on November 15, 2001. The game was ported to Microsoft Windows and Mac OS X in 2003. It was later released as a downloadable Xbox Original for the Xbox 360. Halo is set in the 26th century, with the player assuming the role of Master Chief, a cybernetic-ally enhanced super-soldier. Master Chief is accompanied by Cortana, an artificial intelligence. Players battle aliens as they attempt to uncover the secrets of the eponymous Halo, a ring-shaped artificial world.

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Halo: Combat Evolved is a first-person shooter game in which players primarily experience game-play in a 3D environment from a first-person view. The player can move around and look up, down, left, or right. The game features vehicles, ranging from armored 4×4s and tanks to alien hovercraft and aircraft, many of which can be controlled by the player. The game switches to a third-person perspective during vehicle use for pilots and mounted gun operators; passengers maintain a first-person view. The game's heads-up display includes a "motion tracker" that registers moving allies, moving or firing enemies, and vehicles, in a certain radius of the player.

The player character is equipped with an energy shield that nullifies damage from weapons fire and forceful impacts. The shield's charge appears as a blue bar in the corner of the game's heads-up display, and it automatically recharges if no damage is sustained for a brief period. When the shield is fully depleted, the player becomes highly vulnerable, and further damage reduces the hit points of their health meter. When this health meter reaches zero, the character dies and the game reloads from a saved checkpoint. Health can be replenished through the collection of health packs scattered around the game's levels.

Halo's arsenal consists primarily of science fiction weapons. The game has been praised for giving each weapon a unique purpose, thus making each useful in different scenarios. For example, a charged plasma pistol shot can fully deplete an enemy's energy shield whilst the pistol is one of only two weapons that can instantly kill with a head shot, as long as the target lacks a shield. Halo players may carry only two weapons at once, calling for players to make tactical decisions when managing firearms.

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Halo: Combat Evolved takes place in a 26th-century science fiction setting. Faster-than-light travel called slip-space which  allows the human race to colonize planets other than Earth. The planet Reach serves as an interstellar hub of scientific and military activity. The United Nations Space Command (UNSC) develops a secret program to create augmented super-soldiers known as Spartans. More than twenty years before the beginning of the game, a technologically advanced collective of alien races called the Covenant begins a religious war against humanity, declaring them an affront to their gods. Humanity's military experiences a series of crushing defeats; although the Spartans are effective against the Covenant, they are too few in number to turn the tide. In 2552, Covenant forces attack Reach and destroy the colony. The star-ship Pillar of Autumn escapes the planet with the Spartan Master Chief Petty Officer John-117 on board. The ship initiates a jump to slip-space, hoping to lead the enemy away from Earth.