I believe that what follows will be the Halo 5: Guardians campaign. Be careful: this will be a rather long read.
PROLOGUE: A mix of both trailers. It begins with the trailer that involves the Arbiter and Locke.
Gameplay begins: You look through the cracked visor of the Master Chief, walking and all, when the Forerunner bird-like structure rises througn the sand. Chief grips Cortana's chip, and her famous quote, "don't make a girl a promise, you can't keep," flashes through his mind, followed by a blue frenzy in Chief's H. U. D. that causes the Spartan to fall to the ground. The screen goes to black.
(Queue HALO 5: GUARDIANS title head)
MISSION 1: Sanghelios
Arbiter and Locke are walking through a line of Elites, talking about where Chief could be, when a Brute ship emerges from cloaking. Locke readies his MA5E (next-gen Assault Rifle) while Thel grabs two Plasma Rifles.
(Gameplay begins)
Co-op returns. Locke is player one, player two is the Arbiter, player three and four are the two Elites from Halo 3, aged and battle-hardened.
The team pushes through a platoon of Elites to join the front lines. Brutes drop from pods. However, most are taken out by Sangheili anti-air guns. Others make it in and drop infantry. Players quickly drop these infantry, then fight wave after wave of enemies coming from the grav lift (esque of the Truth & Reconiliation level segment of Combat Evolved) until they can board the ship. The group clears out everything else, and a brief moment of calm is enjoyed.
(Cutscene)As Locke goes through his sack, he finds a Havok bomb. Arbiter strongly protests, but is eventually persuaded by the two other Elites (they are now bots in campaign if not played as.)
(Cutscene ends, gameplay begins again.)
The players reach the control room, clear it out, and arm the nuke.
The team now has 90 seconds to escape through a hole torn into the ship by an AA gun. Arbiter, Locke, and the other two jump out, and the screens fades to white.
MISSION 2: 117
Chief gets up again, and grabs his MA5D rifle (outdated, I know) and aims it at the Promethean Overseer (bird-like thing.)
(Gameplay begins)
The Overseer can take a massive amount of damage, but when it attacks, it reveals it's heart, where it is weakest (I don't care if it's cliche) and can be "boarded." Boarding the Overseer offers a chance to deliver massive damage, but it spawns Crawlers inside to defend it.
Finally, the Chief kills the Overseer, only to find even more Overseers ready to kill him. The Chief sees no way out, until a portal spontaneuosly appears. Chief goes through it, to a room of shifting blue platforms.
(Cutscene)
Chief looks around, but sees nothing. He checks again, and finds Cortana, his trusty partner, in front of him. Chief is baffled for a moment, but eventually accepts it after some explaining from Cortana.
She claims that after the nuke exploded, Cortana disappeared into seemingly nothingness, until a strange hand-like figure grabbed onto her and lifted Cortana up. The A. I. saw a familiar face: the Librarian. The dead Forerunner held onto her, and repaired her at least some using advanced Forerunner tech far beyond even Covenant comprehension.
Finally, John removes his necklace and Cortana goes into it, Chief inserts her into his helmet. He feels a welcoming sensation.
(Cutscene ends, gameplay begins.)
Going through another portal, Chief gets a familiar blue interference, this time weaker and easier to deal with. Chief asks where Cortana was leading them, but Cortana remains silent. Chief asks again, and Cortana gives coordinates to: Earth. More precisely, a ride to Earth. A Pelican lands after Chief and a squad of HAZMAT Marines clear an L. Z. from Prometheans. Chief boards the dropship, the screen fades to black.
(Level ends)
MISSION 3: Predators
(Cutscene)
Arbiter, Locke, and the two other Elites activate their Anti-Grav packs just before hitting ground.
(Cutscene ends, gameplay begins)
They dropped their weapons, so they are resorted to melee weapons (Combat Knife for Locke, Energy Daggers for the Elites.) Moving through the desert, the fighters come across a Brute pack on the prowl (5 infantry, and two Prowlers, with turrets.) The team kills the infantry, and steals their weapons (a new Brute weapon is introduced, the Lancer fires Spike Grenades that detonate on impact, effective anti-vehicle weapons.) After killing the drivers and turret-manners (destroying the Prowlers causes more to spawn,) the crew splits the vehicles, and heads back to the Sangheili city.
(Level ends)
Stay tuned, more to follow.