The game opens with the player and Sergeant Tarkus being deep-striked into Calderis to aid Captain Davian Thule and his initiates. As they drive away the Orks they are confronted by Mek Badzappa and his wartrukk. Although the Mek escapes, the Marines collapse the mine which they are coming through and take a victory. After this they are deep-striked from the cruiser Armegeddon into a hamlet where Scout Sergeant Cyrus is waiting for them. They manage to fight their way to the defender of the hamlet: Sergeant Avitus, and rescue him and save the hamlet from Orks. After this they receive words that Orks have attacked two more key locations and figure they must be being directed by someone. The first attack is led by a Stormboyz Nob named Skykilla and the second is being led by Warboss Gutrencha. After this the Marines obtain information from Skykillas and Gutrenchas and find that both have visited Felhammer Mine recently. They also know that Mek Badzappa is there as well. However Cyrus remains onboard the Armegeddon to provide technical support. When they enter the mine they see a Warp Spider Exarch and his retinue vanish and are rescued from Ork gunners by Sergeant Thaddeus. They also find Badzappa and his Boyz being slaughtered by an Eldar Warlock and a bunch of Guardians. Although they kill the Warlock, the Mek escapes once again. As the Warlock dies he speaks of a greater foe threatening them all but the heroes aren't that concerned.

The Spirit of Fire is sent to Harvest to investigate Covenant activity on the ruined world. Cutter learns from Forge that the Covenant is excavating something at Harvest's northern pole. Upon returning to Alpha Base, the UNSC's main outpost on Harvest, Forge finds it under attack by a contingent of Covenant ground forces. Cutter assigns him to retake the base. Soon after, Forge undertakes a scouting mission and discovers that the aliens have discovered a mysterious Forerunner facility in the snow under the direction of the Arbiter. Forge defeats the Covenant forces before they can destroy the facility and the scientist Professor Anders arrives. Anders determines the facility is some kind of map leading to another star system—home to the human colony of Arcadia. After driving back a Covenant counter-attack on the facility, the Spirit of Fire travels to Arcadia, where the Covenant has begun raiding local cities and slaughtering civilians. Forge makes contact with the local SPARTAN-II special forces, and assists in the evacuation process. The Covenant builds a giant energy shield to hide the construction of a gigantic Scarab super-weapon; the UNSC use experimental equipment to break through. The destruction of the Scarab is a temporary victory; the Arbiter, under orders from the Prophet of Regret, kidnaps Anders. Forge attempts to fight the Arbiter but is defeated; Anders convinces the warrior that she will come quietly if Forge is allowed to live. Forge returns to the Spirit of Fire and the crew follows Anders' transponder signal to an uncharted planet in another star system. The surface of this planet is infested by the parasitic Flood, who attack and consume any sentient life they encounter. While fighting off the Flood the Spirit of Fire inadvertently activates a Forerunner docking station and enters the planet. The crew emerges on the other side and discovers the planet is actually hollow. Here the crew discover the Covenant plan to activate the massive, technologically superior Forerunner fleet inside the planet and use it to obliterate humanity. As the Forerunner ships are powered up, Anders makes a quick escape into a teleporter and is rescued. Cutter decides to destroy the Forerunner ships rather than let them fall into Covenant hands; Anders formulates a plan to destroy the planet by detonating the ship's faster-than-light reactor. The explosion would cause the planet's miniature sun to supernova. Forge and his Spartans are ambushed by the Arbiter and his Elites before they can prepare the reactor; the Spartans finish off the Elites while Forge stabs the Arbiter with the alien's own blade, killing him instantly. The reactor is damaged during the fight, necessitating a manual detonation of the core. Forge volunteers for the suicidal task, telling the Spartans they will be needed in the fight. The Spirit of Fire slingshots around the sun and escapes just as the Forerunner fleet and Forge are obliterated by the planet's blast. Without its reactor, the The Spirit of Fire is left drifting in space. The crew goes into cryonic sleep for long-term storage as Cutter takes a last look at Forge's empty cryo tube. If the game is completed on the "Legendary" difficulty mode, after the credits Serina wakes Cutter and tells him that "something has happened".

Well there is 4 campaigns at the moment 2 German, 2 Allied. Each campaign is told from the specific faction you are currently playing as.