The drop, somewhat predictably, goes horribly wrong. You crash land. Six hours later you wake. You're alone. Night has drenched New Mombasa in a pitch black. The only illumination comes from the flames of burning vehicles and flickering street lights. The Rookie steps out onto the cold urban streets. You see a Covenant patrol - a few Grunts and a Brute - skulking about in search of strays. Immediately ODST reveals itself: this is not the Halo we're used to.
New Mombasa has been designed to be a sandbox, and is in fact the single largest level Bungie has ever created for a Halo game. It's no Liberty City or Capital Wasteland, of course, but it is fully explorable (after about half an hour's play), with no loading required. A curious mechanic, however, breaks up this slower-paced, somewhat stealthy exploration - flashback missions. The Rookie's sole goal is to hook up with his squad. To do this, and to advance the story, he needs to find various objects, remnants of the ODST's actions during the last six hours. When you do, you flashback to before night overcame the city, taking control of one of the Rookie's squad mates and playing out a self contained mission reminiscent of a traditional Halo level. Once completed, you jump straight back into the Rookie's armour plated shoes, and head out in search of the next piece in the puzzle.
The first of these flashback missions triggers when you find the helmet of Dare, Ms Naval Intelligence, as she's described by Buck. Set immediately after drop (each flashback begins with an indication of how many hours after drop it occurs and who you're controlling), you jump into Buck's shoes as he battles the Covenant in Tayari Plaza. Buck discovers dead Elites, apparently killed by Brutes - it's classified, Dare says. We meet a new member of the Covenant family: an Engineer - floating plant-like organic supercomputers forced to wear bombs to protect the military secrets they contain. Dare, guiding Buck via inner ear communication, is in trouble. You go in desperate search of your damsel in distress. As the Covenant close in on her position Buck fears her dead. You eventually find her pod. It's empty. Only her helmet remains...
By the end of the game you'll have stepped into the shoes of all five members of the ODST squad, piecing together the mystery of the last six hours before discovering the truth behind your mission on New Mombasa. The order in which you play these flashback missions is entirely up to you. There's a clear path the game wants you to take in order for it to make the most sense plot wise, one that's encouraged by the Superintendent's (the city-wide AI) less than subtle nudges, but after completing the first two you're free to give the Superintendent the finger and stretch your wings. The only problem is Covenant patrols and Phantom drop-offs have the annoying habit of interrupting your sightseeing just when you don't want them to.
The Rookie, being all human, isn't as powerful as Master Chief. He's not as strong, durable or as fast. He can't jump as high either, although his leaps still have a somewhat unrealistic floaty motion. He can't throw grenades as far, although can still lob them a fair old distance. He can't dual wield, but he can pull turrets out of the ground - an ability Bungie left in for the fun factor despite it making little sense. The most notable difference, however, is that the Rookie doesn't have a shield. Instead he's got stamina. Take damage and the screen will flash red. That's your stamina telling you to get in cover. Take sustained damage and your actual health will deplete. The only way to restore it is to grab a medipack or use an Optican MediGel First Aid station, many of which are dotted around the city. There's no hiding behind a rock and waiting for your shield to recharge in ODST.
The system is a clear nod to Halo: Combat Evolved, which also allowed players to regain health with medipacks, but the gameplay is affected differently. ODST's New Mombasa is more dangerous than any Halo environment ever created, simply because the Rookie doesn't have a shield. In past Halo games, Bungie's defining "ten minutes of fun" mechanic challenged players by virtue of the sheer number of enemies you had to face. In ODST it's different. The number of enemies you encounter at any given time while out and about in New Mombasa is smaller, but because you're much flimsier, leaping in like a madman is likely to get you killed, quickly. Instead, you're forced to pick your fights. You might ignore a Covenant patrol completely, for example. When you're forced into a scrap, when a flashback clue is surrounded by Brutes, Jackals and Grunts, perhaps, it pays to plan your attack carefully, identifying cover, escape routes, sniping positions, flanks and higher ground. It's initially a refreshing change for Halo, and it's hard to think of another FPS set in a futuristic open world urban city.
The Rookie, however, is not completely defenceless. His advanced VISR allows him to outline what he sees, identifying friendlies in green, foes in red and objects of interest in yellow. Essentially sci-fi night vision goggles, the VISR is most useful when exploring the abandoned streets of New Mombasa as the Rookie at night. During the day it makes it nigh on impossible to see what's going on.