Halo 3: ODST, formerly known as Halo 3: Recon, is a First Person Shooter (FPS) video game. It is currently being developed by Bungie, LLC. and will be published by Microsoft Game Studios and is set to be released in the Fall of 2009.
ODST is a campaign expansion and prologue to Halo 3 which will bridge a gap between Halo 2 and Halo 3.
Halo 3: ODST has been known to be in the pre-production phase since January 2008 , four months after the release of Halo 3; and was referred to as Halo 3: Atlas .
Bungie has confirmed that the game will be a first-person shooter, like the previous three Halo titles, and that Christian Allen is working on a completely different project within Bungie. Bungie also confirmed that the player will take the role of a surviving Marine, that the game is set in the ruins of New Mombasa, Kenya; and that it will be released in the US "Fall" of 2009, and that it is the last known Halo project related to the Halo trilogy they are working on.
In another interview with a number of different gaming sources, they revealed key information about the game: ODST will be on a standalone disk, it will "include some new multiplayer maps, some of which will be a part of the Mythic Map Pack," and it will run on the Halo 3 engine. Bungie has also stated that it will be compatible with Halo 3 multiplayer, provided they have the Mythic Map Pack and ODST. Forge, Saved films, and Screenshots will be making a comeback. 
ODST has been described as "returning players to familiar grounds on a vital top-secret mission. ODST's gripping story, co-operative campaign and new multiplayer content will have Halo fans '
reparing to Drop' in Fall 2009". The game is not expected to ship at the full-retail price of $60 as it believed that the game's campaign will only run three to six hours. However, in one of their weekly updates, Bungie stated that the campaign will last between three and five hours. 
ODST has the main protagonist, The Rookie, searching for beacons across the city of New Mombasa . It is up to the player how they get to the specific location, and in what order, giving a very open-world feeling to the game; something for which the main trilogy has been criticized on for linear gameplay. It's likely that these "beacons" are drop pods with a locating device on them. Finding one of these objects sends the player back in the game to find out what happened to one of his four squadmates (Mickey, Dutch, Romeo, and Buck) . Once you go back in time for a short period, you will take control of the player for around thirty minutes, and play the situations that the particular character went through. Unlike the overall game, these flashback scenes go along the path of the linear gameplay that has been common throughout the original trilogy of Halo.
Due to the fact that you are playing as an Orbital Drop Shock Trooper, you will not be using the shielding or the motion tracker featured on the MJOLNIR Battle Armor suits. The Rookie and his squadmates will also lose the ability to dual-wield weapons, such as the M7/Caseless Submachine Gun or the Type-25 Directed Energy Rifle.
On the other hand, you will receive a Personal Digital Assistant (PDA), which will likely give waypoints, mission objectives, and information which will help the player . In addition to the PDA, the Rookie will also have a new "visual mode", which identifies allies and enemies through the HUD on his helmet's visor; as well as a new weapon, the M7S Submachine Gun, which the protagonist lands in Mombasa with during the initial drop.
As stated by Bungie in the January 23rd Weekly Update, the Sangheili will not be an enemy or ally in ODST .
The game is set on the 20th October, during the events of Halo 2, in the First Battle of Earth at Mombasa.
A number of pods carrying ODST's appear in the sky over New Mombasa to clear out the remaining Covenant forces in the city. Prophet of Regret's ship opens a Slipspace portal in Earth's atmosphere above the city, this results in an antimatter-matter annihilation accompanied by a physical shockwave and an EMP. The shockwave tears through Mombasa, badly damaging the city. When the EMP reaches the HEV Pods, it destroys their electronics, which prevents them from making a controlled descent, hence they are destroyed on impact with the ground.
At least one HEV Pod enters the airspace above Mombasa after the primary EMP has passed, but it passed through a secondary residual EMP that partially damaged its electronics. Despite this, it still managed to make a pseudo-controlled descent, albeit a rough one; the HEV crashes through the side of a building, knocking its occupant unconscious. As night draws, the lone Marine wakes up, armed only with a submachine gun, and moves forward to secure beacons scattered across the city.
* United Nations Space Command
o The Rookie - The main protagonist of ODST, and a Lance Corporal in the UNSC Marine Corps.
o The Captain - The Rookie's squad leader, and is a common by-the-books style officer .
o Intelligence Officer - An Office of Naval Intelligence member, and assigned to command the Captain's ODST squad .
o Mickey - One of the Rookie's squadmates, his expertise in the squad is unknown .
o Dutch - The Heavy Weapons specialist  .
o Romeo - The Marksman in the squad, and is quoted as being a "wiseass"  .
o Buck - One of the Rookie's squadmates, his expertise in the squad is unknown .
o Superintendent - New Mombasa's Urban Infrastructure A.I. construct.
o Prophet of Regret - One of the three hierarchs of the Covenant, his only known role in the game is at the beginning where his Assault Carrier enters slipspace over New Mombasa.
* New Mombasa, Kenya
o Lumumba District
o Mbaraki District
o Tanaga District
* United Nations Space Command
o M7S Caseless Submachine Gun
o MA5C Individual Combat Weapon System
o BR55HB SR Battle Rifle
o M90A Close Assault Weapon System
o SRS99D-S2AM Sniper Rifle
o M41 SSR MAV/AW Rocket Launcher
o W/AV M6 Nonlinear Rifle
o Type-2 Energy Weapon/Hammer 
o Type-25 Carbine
o Type-25 Directed Energy Pistol
o Type-25 Grenade Launcher
o Type-33 Guided Munitions Launcher
o Type-33 Light Anti-Armor Weapon
o Type-51 Carbine
* United Nations Space Command
o Single Occupant Exoatmospheric Insertion Vehicle
o M12 "Warthog" Light Reconnaissance Vehicle
o M831 "Warthog" Troop Transport
o D77H-TCI "Pelican" Dropship
o Type-25 "Wraith" Assault Gun Carriage
o Type-52 "Phantom" Troop Carrier
o Assault Carrier (Appearance only)
Main Article: Keep It Clean (Trailer)
First, different screens of areas of New and Old Mombasa come on screen, followed by HEV pods entering the atmosphere. Here you can see the Prophet of Regret's carrier on the right, then it moves behind buildings. The pods come into view as a large explosion (Caused by the Prophet of Regret's carrier entering Slipspace) appears, sending the pods off-course. Then, the Superintendent appears, and shows warning signs and then reboots. All the screens from earlier flash onto the screen, but heavily damaged from the blast. Then, a single HEV pod falls through the sky, when the Halo 3 Logo appears. The pod then crashes into the camera and goes black, followed by the words ' Prepare to Drop'.
The second part to the trailer, released on the 8th of October, followed on from this - the Superintendent reinitialized itself, attempting to assess the catastrophic devastation and tries to identify the object that survived. Later, after a Covenant Phantom dropship has passed by, the object opens to reveal a lone surviving Marine Shock Trooper to clamber out, wielding a modified M7S Caseless Submachine Gun. The Marine narrowly avoids a Jiralhanae patrol, before encountering messages from the Superintendent, presumably leading him to safety.
Though ODST lacks an official multiplayer mode on Xbox LIVE, all map packs (Heroic, Legendary, and Mythic) will be available for those who haven't already purchased them over Xbox LIVE Marketplace . In addition to the other map packs, an additional map pack will be featured in the ODST package. These maps are currently known as Citadel, Heretic, and Longshore.