Mombasa Streets

Mombasa Streets is the first campaign level of Halo 3: ODST, and serves as "the hub", an open-world environment used to access flashback missions. It is set in the city of New Mombasa.

Summary
Unique as a Halo game level, this level is an open world environment, which contains "beacons", objects left by or noted by the Rookie's squad members, which trigger "flashback" levels, explaining previous events. In addition to the beacons, the level also has additional content, such as Audio Logs and Supply Caches.

The city map provided by the Superintendent highlights points of interest, such as the "beacon" objects. The Superintendent also guides the player into the right direction using the city's plentiful digital signs and billboards.

During the first playthrough, some areas in the city are inaccessible, and the player is forced to play the flashbacks in chronological order. Completing flashbacks unlocks more areas in the city, allowing the player to progress to the next beacon. Once game is completed, the city is fully unlocked for exploration.

Plot
The level starts six hours after The Squad's failed orbital insertion, in the occupied city centre of New Mombasa. Some time after the Rookie wakes up, exits his pod and starts to explore the city, the Superintendent will contact him via a public phone and upload a map of the city to his VISR navigation database. After finding the final beacon and completing the last flashback level, the Rookie will receive a distress signal from Captain Dare, who is pinned down in the data center under the city. At this point, the city will be mostly locked, with only a way to the nearest underground entrance, where the Rookie will descend into the depths of the city.

Transcript
Text appears on black screen:

The Rookie

Occupied city center

6 hours after drop

''{The Rookie slowly opens his eyes in his pod. There is static interference in his visor, and multiple cracks in his pod's windows. The screens that are still working are filled with static. A Covenant Phantom flies in the background.}''

''{Shortly after the dropship passes, the Rookie activates four bolts to release his pod door open. Unknowingly, the Rookie hops out, only to fall about 10-20 feet to ground, injuring him slightly.}''

Beacons
There are six beacons around the level, each an object related to a squadmate. The following is a list of the locations of these beacons, and the flashback levels they trigger, in chronological order.

Trivia

 * This level is the only level in Halo 3: ODST which does not unlock an achievement upon beating it on Normal or above. This is probably because of the fact that this is a free roaming level.
 * This is the first non-linear open-world level ever in a Halo game, and the largest open environment Bungie has ever created.
 * This level is the easiest to avoid enemies, as there are several detours and wide areas allow you to sneak past enemies. Just stay near walls to avoid being spotted by a sniper.
 * The level's moody atmosphere, from color palette to lighting and music, is said to be influenced by the Film Noir genre.
 * Numerous dead Elites can be found on the map, often alongside dead Brutes, indicating most of them may have been killed by the Brutes. Areas where they are found are usually highlighted with Superintendent's symbols.
 * There is an abandoned Elephant around the city.
 * past the abandoned Elephant, there is a deceased Elite without shoulder pads. Due to the frequent appearances of Elites without shoulder pads and the fact that they only sport white/silver armor, it is possible that they could be ranked as either Stealth Elites, Ossoonas, or Sangheili Ultras.
 * On this level, there are several graffiti markings where you can read BELIEVE, maybe being a reference to the marketing campaign for Halo 3 of the same name. There is other graffiti in the city, some of which read "YOU LOOT, I SHOOT", "KINSLER IZ A THUG","UNSC=LIERS", "REMEMBER REACH", and "GLASS THIS!".
 * Within one of the many building enclosed courtyards, the player can find two lone Huragok floating around. From what can be observed, they could have possibly strayed from their patrol and have yet been discovered since all of the units in the courtyard are deceased except for them.
 * If a group of hostiles is giving you trouble, fall back through the bulkhead to the last area. They often disappear, or forget you were there. This is very useful for patrols protected by Engineers, when you can't get a shot at it for the volleys the brutes are laying on you.