Now this is what Mass Effect 2 is all about. This excellent add-on has great action, a smattering of memorable moments, and a final sequence that's unlike anything you've seen in the series thus far. Overlord is a short but sweet microcosm of what makes the main game so entertaining, spiced with a neat hovercraft and a few quick puzzles to vary the tempo. The vehicular sections are a missed opportunity, and some of Mass Effect 2's occasional glitches have carried over into the latest content. But these minor gripes barely lessen the dramatic impact of Shepard's latest mission--which also happens to be one of his (or her) best.
Mass Effect 2: Overlord begins with a summons to a newly marked planet on the galactic map, where Cerberus has been conducting some rather questionable--or perhaps honorable, depending on your viewpoint--experiments. The problem? A chilling virtual intelligence has sequestered itself in a fortified research facility, and the project's lone surviving scientist warns you of potentially dangerous consequences if you don't stop this malevolent entity from spreading its influence across the galaxy. And Overlord does a terrific job of impressing upon you the maliciousness of this viral consciousness. The VI's digitized groans that plague you during your adventure are absolutely chilling and only grow more so as you begin to understand its growls. Haunting new music instills a palpable sense of tension, and the dark interior spaces provide a sinister contrast to the shimmering waterfalls and scorching lava rivers on the planet's surface.
You roam the surface not on foot but in the Hammerhead vehicle first seen in the free Firewalker add-on. This hovercraft feels appropriately floaty, allowing you to ride large gusts of steam to higher ground and making it a breeze to zoom to your next destination. The Hammerhead is used mainly as a quick mode of transport, and flitting about in it while gawking at the idyllic scenery makes for a nice change of pace. You also take aim at a few sentry guns and engage a large turret in a boss battle of sorts, but the overall lack of vehicular action makes the Hammerhead portions feel mildly unfulfilling. Fortunately, the core on-foot action is some of the best in the series. A few of the larger environments let you take on Geth and other foes from multiple angles, which are a nice change from the straight-on encounters that typify most of Mass Effect 2's battles. Screen-shaking explosions make one battle particularly fun; a surprise appearance of Geth foes via elevator leads to another enjoyable skirmish. This isn't Overlord's only important elevator sequence, however. A ride in a shaky lift is one of the add-on's most memorable and frightening moments, and you don't fire a single shot.
You may notice some of Mass Effect 2's annoyances creeping into the action here. The cover system can still be a little buggy, causing you to jitter in and out of cover when you just want to pop out to take a shot, and your two AI teammates aren't always the brightest bulbs. But you'll probably forget these quirks once you reach the exciting final sequence, which features an art style you haven't seen in this series and an enjoyable boss fight that surpasses Mass Effect 2's uninspired final encounter. This impressive sequence leads to a bittersweet ending and features some uncomfortable images that will linger in your mind even after you've returned to the Normandy.
Mass Effect 2: Overlord features a nice mix of gameplay elements, punctuating shooting segments with scripted set-piece events and smooth vehicular exploration. You even solve some light puzzles in between exciting encounters and tension-building travel, such as one in which you use a console to move platforms about so you can cross to your destination. It may only last 90 minutes or so, but at 560 Microsoft points ($7), this is one downloadable delight Mass Effect 2 fans shouldn't miss.