Mass Effect is about many things - exploration, war, adventure, romance, politics and, of course, lots of weird and creepy space aliens. But above everything else, Mass Effect is about choice. Watching the game for just half an hour at this year's E3, we were positively staggered by the number of large and small decisions the player was forced to make, and how reaching an impact each one had.

It begins before you even start the game. Do you want your hero, Commander Shepard, to be male or female? The selection affects more than your appearance, which can be customized to an infinite degree anyway. Playing a woman will alter certain chapters of the storyline and, because a different voice actor was needed, affect the interpretation and emotion of every scene.

The options for intergalactic travel are equally daunting. You browse the Milky Way with a 3D holographic map, one so packed with stars and planets that the screen becomes almost blinding. Some of those worlds are integral to the plot, many contain treasure or side quests (collecting ancient Turian flags, for example) and others are completely uninhabited. Regardless, you can choose to visit them all.

Our demo took us to a watery world known as Virmire in response to a garbled transmission. We constructed a four-man away party from a number of possible team members, double checked everyone's weapons and armor, jumped into a sturdy all-terrain MAKO vehicle (complete with jump jets, machine guns and zoom camera), dropped out of our ship's hull and splashed into the waves of the surface below.

Turns out, Virmire isn't quite the tropical paradise it first appears. As soon as you hit the shore, you're ambushed by enemies and must dispatch them through a combination of gunfire, team tactics, cover and - best of all - biotic powers. We watched as Shepard used telekinesis to lift a creature into the air and smash it against a nearby wall; he could also freeze and then shatter them with bullets.