There are massive spoilers in this, In fact I feel like I really shouldnt be posting it.
The beginning of the game is a cutscene of the Forerunners (although you don't know who they are at the time), heading to Earth. The Forerunners are an interesting looking, humanoid creature, but their entire bodies and faces are covered in silver armor, so their physical features remain somewhat hidden. They're crashing into a planet, as their ship has been attacked by the Flood, which is later revealed to be Earth. Only one survives, but he carries a code and vial to a certain part of the planet (which is lush and green). There, he unlocks a structure buried within the planet's surface and descends below.
In the first level, Master Chief has to escape from the Forerunner ship that the Prophet of Truth is taking to Earth, whilst it flies through the planet's atmosphere. The ships lands in a desert near New Mombasa, and lands as though it is locking itself upside down. The next two levels take place through this desert area and catacombs below it.
The scene then switches to the Covenant homeworld, Pontiff Quarum. The planet glows with a black/purple colour, and has many holes in it leading down into cities; the actual surface of the planet acts as an energy shield. The Arbiter is shown, who is on trial, being beaten up by an Elite who is guarding his cell, as he has opposed the will of the Prophets. The Arbiter, Miranda Keyes, and AJ Johnson returned to Pontiff Quarum to try and stop the Covenant fleet from going to Earth and activating the Halo rings from the Ark. The Arbiter has been given a hearing in front of the Prophets of the Covenant homeworld, in which he explains the information he has gathered from Cortana and Penitent Tangent. The Covenant do not believe they are telling the truth, and so they are put in prison, even though they have the backup of Penitent Tangent, the Monitor of Delta Halo. The Prophets state that their clairvoyant Prophet, the Prophet of Foresight, believes that before the Great Journey begins, the Forerunners will rise again to assist with it.
Abecedarian is a young Elite who breaks the Arbiter free from prison. It turns out, that he is the Arbiter's son. They travel through the streets and the sewers of the city on Pontiff Quarum they are in. Eventually, they meet up with a group of loyal Covenant who believe their story, and rescue Keyes and Johnson from their imprisonment. They bring them to a port in the city, and the Arbiter takes controls of a Phantom. The last level of this sequence allows you to fly the Phantom over the Covenant landscape and out into space. This is the first time in the Halo series that involves a space-shooter scenario, and it is meant to be great fun.
Next, there is a cutscene of Cortana talking to Gravemind. She learns that Gravemind is the single consciousness of the Flood, and therefore controls their actions. It tells her about the history of the Flood and the Forerunners, and she learns that the Halo rings are more than just weapons, and that the Forerunners are back to reclaim control of the galaxy. She learns that there is also more to Earth than meets the eye.
Back on Earth, Master Chief attacks a group of attacking Covenant, but is badly injured. He retreats to a nearby, surviving part of Kenya where he meets up with some UNSC Marines defending the area. The group make their way through an area of jungle and plains, and end up in a city which is still under siege by the Covenant. They go through the streets and scale a skyscraper. Master Chief gets back down by running along nearby rooftops, and finally arrives at the HQ setup at the far end of the city. This level seems very similar to the E3 2003 demo.
All the returning weapons from Halo: Combat Evolved and Halo 2 are being completely redesigned to fit into the game.
There is a weapon very similar to a SAW, or the turret on the back of the typical Warthog. It is held over the players shoulder and appears at the top right of the screen rather than the bottom right. It is very powerful, and whilst firing, you can see the bullet chain hanging down and going into the gun.
The Plasma Pistol is nothing like its Halo: Combat Evolved and Halo 2 counterparts. The charge-burst homing ability has been greatly decreased and it fires more like the Magnum.
There is a Forerunner staff-like weapon that emits a very powerful beam of light.
The Jackal Shield can be dual-wielded in both the campaign and in the multiplayer.
The Flamethrower can be used in the campaign.
In the campaign, the Forerunners use simple weapons that have the firing magnitude of the Scarab's cannon. If you've found the Scarab Plasma Rifle on Outskirts in Halo 2, you'll know what to expect.
The SMG will remain as the standard 'usual' weapon.
You will not be able to dual wield Energy Swords.
There will be different variations of the Warthog, but I am unclear as to which. Possibly the Snowhog and the Transport Hog that were both removed from Halo 2.
There will be a human flying vehicle that has a similar design to the Warthog.
There will be a new Covenant vehicle, that will most likely be a replacement for the Spectre.
There will be a Forerunner flying vehicle, the Leviathan.
In the campaign, you can fly the Phantom.
In the campaign, you can fly the Longsword Fighter (the ship Master Chief gets in at the end of Halo: Combat Evolved).
As far as I am aware, all the other vehicles will return.
The Multiplayer Maps
Zanzibar will return from Halo 2. It will be mangled and destroyed, as if it has been in the middle of warzone. The sky will also be red, as if it is on fire, to reflect this. The fan in the middle will be torn to pieces, but it will still be working to some degree.
The best map from Halo: Combat Evolved will return for Halo 3. This time, however, it will have four branches leading off of it instead of two, meaning it looks more like an 'X' than a 'V'.
Hang 'Em High
The second map from Halo: Combat Evolved that is returning. A large tree in the middle of the map provides access from one base to another via its branches. The bases are more defined now, and the location looks as if it is suffering from planet overgrowth.
This map is a new map, and may be the biggest in the game, as it is 3-4 times bigger than Waterworks. It is basically an entire city. It is shaped like an arrowhead, as the name suggests, and the tip of this is a rocky area on a beach. Past that is a rundown district containing car shops and a church. There is a river that cuts through the middle of the city, with bridges that allow you to cross over it. There is also a metropolitan area with huge skyscrapers and a bridge/lift system that connects some of them. A few of these bridges are glass walkways, and can be destroyed when shot or hit. There will probably be some fall damage in the game based on this, but probably not to the extent of Halo: Combat Evolved. There are warehouses, a military base, and even a coal quarry with caves and tunnels. If Bungie can pull this off, Arrowhead will be the single greatest map ever made.
This is a reasonably large map and is asymmetrical, and set in the redwood trees. The branches are massive and allow you to cross between trees, along with makeshift bridges. Falling off the trees is certain death. Also, this map takes place at night.
The third original map is around the size of Coagulation and is based on an island shaped like a 'C' near one of the Halo-Earth ring's expansion points. As though cutting the C in half, in the middle of the island is a huge wall that cannot be crossed (there are no flying vehicles on this map either) without engaging a ramp. Most of the island is flat, but at the tips of the C there are high rising cliffs with tall towers on them. Connecting these two towers and stretching out over a lagoon that separates these two parts of the island are energy bridges that lead to a central platform. On this hovering platform is a control panel that allows the wall to be opened or closed on one side (whichever team activates it first will have their gate open). After these gates are opened, it is possible for the team with the open gate to go up a ramp, to the top of the wall, and therefore to the other team's side of the island. Only one of these gates can be open at one time. The hovering platform also has a third energy bridge which can be activated to allow access between itself and the wall.
Master Chief a.k.a. John - Spartan 117
This is the last time Master Chief will have to stop the Halo rings from activating. This time though, there is more than that threat, Master Chief has to defend Earth also from the Flood, the Forerunners, and a Covenant invasion.
Captain Keyes' daughter fights alongside the Arbiter, Cephaus, and AJ Johnson in this installment of the Halo series, defending Earth from the Covenant, the Forerunners, and the Flood, after being rescued from the Covenant homeworld.
AJ Johnson a.k.a. Sgt. Johnson
Johnson is in the squad with the Arbiter and Miranda Keyes in Halo 3. He fights on Earth after being rescued from his prison on Pontiff Quarum.
2401 Penitent Tangent a.k.a. Delta Halo Monitor
Penitent Tangent is the Monitor from Installation 05 (Delta Halo), and tries to help persuade the Covenant to stop their invasion of Earth on Pontiff Quarum.
343 Guilty Spark a.k.a. The Oracle
343 Guilty Spark may return, but I am unclear on this matter. Considering he was present at the end of Halo 2, telling Keyes, Johnson, and the Arbiter about the Ark.
Cortana spends most of the game coversing with Gravemind, discussing the Flood and the Forerunners. She can transport to any of the Halo rings via its teleportation connections, and therefore to Earth, using the Halo-Earth ring as a destination.
Gravemind is the sole consciousness of the Flood, and controls everything that they do. He tells Cortana about the Halo rings, and uses his Flood legions to try and destroy the Forerunners. He spreads as the Flood do, and is therefore present wherever they are.
The Arbiter, in this game, fights alongside Keyes' squad, Cephaus, and Master Chief. His son Abecedarian saved him from his captivity on Pontiff Quarum and helped him to go to Earth and save the universe.
Abecedarian is a young elite, the Arbiter's son, and breaks him free from jail on the Covenant homeworld.
Cephaus is the Forerunner that activated the Halo rings, killing most of the Forerunners, millions of years ago. He has watched the human race evolve and live since then, and decides to defend them during the game.
Lord Hood is the general of the UNSC forces, and is trying to fend off the Covenant from Earth.
The Prophet of Truth
Truth leads the Covenant forces on Earth, and is using the Forerunner ship to activate the Ark, and therefore all the Halo rings.
The Prophet of Foresight
This is the Prophet that condemns the Arbiter to prison on Pontiff Quarum, as he does not believe his claims about the Halo rings, and believes that he is opposing the will of the Prophets.
The Forerunners are an ancient race that created the Halo rings, and activate them to destroy the Flood. They were previously believed to all be dead, save from Cephaus, but, using Halo as a time machine, some of the Forerunners escaped to the future. That future, is the present in Halo 3.
The Flood are attacking Earth along with the Covenant and the Forerunners in Halo 3, but are also fighting against those other two armies. They are controlled by Gravemind, which acts as their brain.
As far as I know, three of the main characters are killed during the course of the story. I won't tell you which.---
:EDIT: I have recieved a reply from my friend as to where his sources are. It turns out he copied and pasted it from http://www.freewebs.com/jaxx931/halo3.h
SORRY FOR THE CONFUSION