Lieutenant Major Harry Jackalson was a soldier serving the UNSC, in his long years of combat he fought hordes of alien invaders known as the Covenant.
Eventually one mission had him searching for several teams and the entire medical science staff went missing on Barren Planet-1042684, a big planet was supposed to be colonized for control of the human populace but meet with forces unknown.
Jackalson fought uglies such as the "Pfhor" and the "Fallen" whom they all somehow came out of nowhere and began not only shooting at him but also at "each other!"
In the middle of the fight Jackalson found ancient ruins from an unknown civilization, when he underneath the deep ground, he discovered "THE FLOOD ZOMBIES!!!"
Out of haste, Jackalson quickly began setting off the self-destruct fail-safe charges and left the already ruined planet. As Jackalson escaped, he was attacked by Flood Bug and killed it. He crash lands on Planet Mondo and escapes alive (strangely), but unaware that he was infected by the Flood Bug.
He sought help from fellow UNSC Troopers only to reveal that they're also come under attack by unknown alien forces not related to the Imperial Covenant Empire. Jackalson rallies the survivors and aids them in their battle against the evil aliens of Pfhor, Fallen, Vex, and the Hive, even the Covenant (during these bad times) joined the fray and became outright chaos.
Suddenly, Jackalson looks up in the skies and discovers space-faring starship vessels that was once belonged to both UNSC and Covenant, and horrifically realizes that the "Space Zombies" are coming for him.
Fight against this struggle, the multi-aliens stop their attacks upon each other and began shooting at the space ghouls. Jackalson and his human team were intending on the same thing only for Jackalson to discover that his human friends have turned into Flood Zombies!
Grieving and devastated, Jackalson kills his former friends and blows them into pieces so they can no longer suffer anymore.
Jackalson heads to the valley solo, and finds the aliens up in open arms, killing each other as chaotic battle rages all over Mondo and the presence of the Space Zombies makes things a whole lot worse, as more zombies are being crafted by the Flood Bugs and Jackalson is hellbent on finding the source of the Flood Bug infestation.
Jackalson finds an infected war carrier above him, he immediately steals a Banshee to sneak right inside, battle countless hordes of the infected and eventually faces off against the Flood Boss HiveMind.
Using every gun from his hammerspace arsenal inventory, Jackalson tries to destroy the Flood Boss HiveMind, only to make him "ANGRY!", the Hive Boss defeats Jackalson with one swoop of his tentacle, and as Jackalson is on his dying breath he said that the zombies will never win.
Only for Hive Boss, say that Jackalson was the Flood Zombie that brought the space zombies onto Mondo as a parasitic beacon for their arrival. Shocked and distraught from this belief as Jackalson died and became a Flood Zombie.
7 years later, the Flood have take over Mondo, as the surviving humans and aliens are battling the Flood Zombies to no avail, but eventually signs of hope came to them in form of a space-warp portals appearing out of nowhere as in one side UNSC starships arrive with Spartan reinforcements while the other side have Jumpships coming out of their side of the warpgates.
As the smaller ships land, the Spartans and Guardians began battle the Flood Zombie hordes and surpressing the parasite from spreading their virus.
However, despite the calling of the storm's passing, the alien xenos began drawing their attention back at humans (besides each other), and an all out war has eventually begun on Mondo, humans of the Haloverse and the Guardians of the Destinyverse make a desperate team up of battling the hostile xenos as more civil lives are being threatened to the point of extinction.
Post-credits scene shows the teaser trailer of the Cyborg Enforcer from Marathon making his/her debut in the Bungieverse.