From Halopedia, the Halo wiki
It is the first level in the Halo Wars campaign to feature the Flood. After the UNSC Spirit of Fire finishes its slipspace travel to pursue a Covenant ship holding Professor Anders, they enter into orbit around an unknown planet, and as Captain Cutter orders Sergeant John Forge to the surface, they encounter units of new Covenant species, the Brutes. As Sergeant Forge's Elephant crew kills all of the Jiralhanae, they are assisted by Spartan Red Team. Two other Elephant crews on the surface, Echo and India, however, reported attacks from unknown hostiles, before all contact from them was lost.
As Sergeant Forge's units and Spartan Red Team investigate, they eventually encounter the Flood, and are forced to destroy and clear any Flood forms surrounding the two other Elephants. Then they are ordered to bring all the three deployed Elephants back to a nearby firebase.
On Arcadia, Professor Ellen Anders records the Arcadia Forerunner ruins and destroyed Super Scarab with a handheld camera while Sergeant John Forge horses around. Suddenly, the two are attacked by the cloaked Arbiter who bests Forge in a fight and wounds him. Anders stops the Arbiter from killing Forge, promising to go quietly in exchange. As the two depart in a Covenant dropship, Red Team arrives in a Warthog in a failed attempt to stop the Arbiter's departure with Anders. Retreating to the UNSC Spirit of Fire, Forge reports Anders' capture while Serina tracks her transponder signal which quickly goes to lightspeed. With little time to make a decision and at the urging of Forge and Jerome-092, Captain James Cutter orders the Spirit of Fire to pursue.
On February 23, the Covenant vessel emerges from slipspace followed by the Spirit of Fire in uncharted space above a planet. Forge, Red Team and Marines are sent down with Elephants to secure a base, coming into contact with Covenant forces, including Brutes. As the teams clear the area, Recon Teams Echo and India come under attack by the Flood. Forge and Red Team must attempt to rescue Echo and India and retrieve their Elephants to bring them and Forge's Elephant to the newly constructed base.
Fades into Professor Anders and Sergeant John Forge, who are at the remains of the destroyed Type-29 Scarab. Anders holds a small hand-held camera, documenting the Scarab's remains. Forge grabs a piece of debris and poses with it.
Forge drops the piece and turns his back at Anders. Anders steps forward and continues observing, but finds a camouflaged entity in front of her. Almost immediately, the invisible entity grabs Anders by her throat and uncloaks.
Forge draws his M6C magnum and points it at the Arbiter.
Arbiter drops Anders and moves towards Forge. He shoots the Arbiter with his pistol but it has no effect. The Arbiter activates his camouflage, moves behind Forge and cuts his Pistol in half with a swing of his Energy Sword. Forge falls down and the Arbiter moves to finish him with a stab. Anders runs in between them.
The Arbiter grabs Anders by her throat and walks into the Gravity lift of a Spirit dropship hovering above. The ship flies away as the Spartan Red Team arrives with a Warthog firing at it. After the ship goes away, the Spartans get off the Warthog and secure the area. One of them approaches Forge, who is still lying on the ground from the attack.
A Pelican flies towards the Spirit of Fire.
Forge enters the bridge with Jerome-092 behind him. Captain Cutter leans over the holographic display of Arcadia. The two stands beside him.
The holo-table shows a red dot, a representation of Anders' signal, heading away from Arcadia's hologram and a green hologram of the Spirit of Fire. It is leaving Spirit of Fire's Close-In Weapon System (CIWS) limit and approaching the Long Range Weapon System (LRWS) limit.
Cutter pauses momentarily, then finally makes a decision.
Forge and Jerome leave while Cutter approaches the front of the bridge. Hatches begin to seal around all the windows as the ship prepares for slipspace travel.
Cuts to black.
Fades in to two ships coming out of slipspace in orbit, as seen in the atmosphere of an unknown planet. A piece of the landscape twitches, then turns around towards the camera as three small tentacles twitch around its chest, revealing an unknown lifeform that resembles a Sangheili.
FEBRUARY 23RD 2531
Cuts to Spirit of Fire's bridge, Cutter is looking from the front windows as hatches unseal around the windows, revealing a red planet.
Cutter pauses, sighs and turns around, walking towards the bridge's door.
Captain Cutter exits the bridge.
Two M312 Elephants with two squads of marines each move though the terrain.
The teams are revealed to be very close to a heavily fortified Covenant position, where two Spirit dropships just flew away from.
After enough Marine squads are trained, Forge and the Marines move towards the beacon, running into Covenant forces.
If an Elephant takes some damage from the Covenant attack:
If an Elephant takes heavy damage:
After a bit of heavy fighting, a Pelican drops off Spartan Red Team behind Covenant positions.
With the help of Red Team, the Covenant forces are eliminated.
They regroup near the beacon.
Several groups of squid-like creatures approach the Marines. The creatures latch themselves on the Marines and the Marines fall on the floor.
Echo Team suddenly disappears on map and radar.
Very soon, Recon Team India reports in.
Several groups of the same squid-like creatures approach the Marines, and the Marines suffer the same fate as that of Echo Team.
India also disappears from the map and radar.
The UNSC forces move close to where Recon Team Echo was stationed. A red flare is on the ground.
They reach Echo Team's last known location...
Many of the small squid-like creatures crawl over the ground. Echo Team Echo's Elephant is covered with giant biomasses and large tentacles. Marines from Recon Team Echo lie on the ground, lifeless.
The small squid-like creatures crawl over the dead Marines, and the bodies stand up, mutating.
The UNSC forces destroy the Flood forces surrounding the Elephant.
The UNSC station and its base turrets have been set up.
If a lost ODST squad is found:
If another ODST squad is found:
If the third lost ODST squad is found:
At the last missing ODST squad:
The entire ODST squad turns into Flood combat forms. The UNSC forces have no choice but to kill them.
With a stronger UNSC force, Sergeant Forge heads out to recover the second Elephant, and succeeds.
All Elephants are back at the base.
The three Elephants lock down beside the UNSC base.