The demo contains the very first mission from BioWare's celebrated sci-fi RPG sequel, where Commander Shepard must escape a besieged space station, as well as a later mission where our intrepid hero recruits the charming Subject Zero to zir rag-tag crew.

Neatly, Commander Shepards created in the demo can continue their adventure in the full game with their experience, story decisions, achievements and weapons intact.

As for the 'Overlord' downloadable content originally pack due alongside the demo, that is still scheduled for tomorrow. However, publisher EA has confirmed that the $7 (560 Microsoft Points) DLC will be released simultaneously on both PC and Xbox 360.

If you want to try the demo but didn't play Mass Effect, BioWare has created the following trailer narrated by the kindly Martin Sheen to explain what you've missed.