I'll let the excellently information-packed D+ website tell you itself:
So what exactly is Darwinia+?
Darwinia+ is Introversion’s fifth game release and its first venture onto consoles. Darwinia+ has been created for Microsoft’s Xbox Live Arcade service. Darwinia+ will be a fantastic bundle combining Introversion’s award-winning classic hit Darwinia and the action-packed multiplayer follow-up, Multiwinia. With a highly unique blend of stunning retro-arcade visuals and explosive real-time strategy gameplay, Darwinia+ is a lovingly put together homage to the halcyon days of gaming that has gained cult status across the games industry.
Tell me more about the story…
The world of Darwinia is a virtual themepark running inside a computer network built by a computer genius called Dr Sepulveda. Darwinia is populated by a sentient, evolving life-form called the Darwinians. They are the product of a decades worth of research into genetic algorithms and their intelligence has been evolving for thousands of generations.
Unfortunately the systems of Darwinia have been overrun by an evil red viral infection which is multiplying out of control and must be stopped. The gamers task in Darwinia is to destroy the viral infection and save the peaceful Darwinians from extinction.
In Multiwinia, the once-peaceful, loving Darwinian nature has turned dramatically for the worst. Having successfully fought off the red virus and gained greater awareness of their autonomy, the descendants of the Darwinians, the Multiwinians, start to disagree on how things should be run and begin to split into opposing factions, all of which are aggressively competing to share the limited Darwinian world resources. Tribes of Multiwinians now roam the fractal landscape hell-bent on fighting each other in order to gain world dominance. It is, quite literally, survival of the flattest.
Get it? Got it? Good. Now go sit by your 360 and wait patiently...