ODST was an originally an expansion pack that spiralled out of control under the weight of its own ideas. It's a very different breed of Halo in many ways, with the dark moody vibe and interesting storyline jarring with the usual colourful big action. An engrossing meta-story can be unearthed by finding audio diaries scattered throughout the explorable non-linear city, which yields serious amounts of exploration without ever feeling like a chore. Note, however, that you'll need to have played Halo 2 to really understand (or care, more importantly) about what's going on. Thanks to RS2010 at Hot UK Deals.