Ask someone, anyone, to summarise the events of the first Mass Effect game and they'd have to retire to a well-lit room for a couple of hours, to put together a long document outlining a number of options of where the hell to start.
BioWare's sci-fi universe was exquisitely rich and detailed in story, and pleasingly deep in the relationships that you built with your crew. And thanks to the quality of the script and voice acting, even the minor characters rarely slipped into easy cliché.
Set in a huge universe, in which it genuinely feels possible to finish the game with a huge percentage untouched, Mass Effect 2 is a very similar game to its predecessor. The six classes spread among the three specialities still have the same names, but have been adapted: Engineers move away from their healing role (health regenerates, now), and can command combat drones to distract enemies. Biotics have had their slightly overpowered Lift skill replaced with slightly more balanced crowd control powers, and Soldiers - well, they still shoot enemies with guns.
Everything's been shuffled around slightly to prevent it being too familiar, and the result is more fluid and intuitive combat - ineffective powers are tinted red, preventing you from wasting time. It's also more visually fluid, in terms of the game's improved frame rate.
Working alongside the regular levelling up and team-building rewards for missions, is the familiar progress along Mass Effect's trademark conversation trees. Be a pious hero, and you'll develop along the Paragon branch, unlocking new ways to be ever more lovely. Or, as the game puts it, "become the ultimate badass" by following the Renegade path, which usually involves hurling accusations, saying you quite like genocide, and meting out justice of the brisk and deadly variety. This system also lets you intervene in some cutscenes, to change the outcome of a conversation. For a fairly bipolar morality system, it still manages to be stylish, and both sides are written well enough to fit into Shepard's believable range of actions.
Let's talk about what fans of the first game care about: the plot. After an ambush, the opening scenes of the sequel see Shepard walking along the outside of his freshly-destroyed starship, the Normandy. His attempt to save Joker fails, as a huge laser shears him from the vessel and he swirls into the atmosphere of a nearby planet. If you're going to have your hero reset to level one - which he is - you might as well get slaughtered in style.
So, Shepard is recovered, restructured, and resurrected by Cerberus, a secretive pro-human organisation who might - or might not - be a well-spoken, more ambitious, and better-funded version of the BNP. Under the instruction of the charming and morally ambiguous Illusive Man, Shepard is provided with a similar Normandy, and resumes the business of assembling a crew to save humanity. Each crew member has his own personal journey that's handled with a wonderful lack of sentimentality.
The temptation to blurt and gush here is intense: but it's not our place to rob you of your own journey. Safe to say: the Reapers are still at large, the effects of the genophage are keenly felt, and thousands of human colonists are getting paralysed by swarms of insects, and abducted by a mysterious new threat, the Catchers.
The streamlined combat gives you flexibility to play as you see fit. It's difficult, but possible, to play the game as a traditional FPS lone wolf - fallen teammates are automatically revived when the battle lulls, allowing you to save medi-gel. But the game can slip quickly into repetition. It's much more rewarding - and feels more correct - to take the hands-on approach.