Without our intervention, organics are doomed. We are your salvation.
Downloadable content for the Mass Effect series frequently puts fans in an awkward position. On one hand, most paid DLC for the title expands on the story but only after most players have seen the ending. If these extra plot points were so important, then why weren't they in the game in the first place? On the other hand, as much we can't argue against free multiplayer content or the optional Extended Cut, we want DLC with more substance, more meat on the bone. That's what Mass Effect 3: Leviathan aspires to give us.
Once Shepard has access to transmissions in the game, this DLC begins with Admiral Hackett telling Shepard of a suspicious rumor about a certain Leviathan that has been able to fight off a Reaper. Perhaps this is a particular beast like a thresher maw or perhaps it's an ancient weapon capable of incredible power. Whatever the case may be, it's important enough for Shepard to investigate.
After exploring the Citadel for any additional information on the rumor, Shepard soon arrives at a mysterious planet, likely yet to be explored on the star map. In the off-hand demo shown at EA Redwood Shores, Shepard and his team (in this case, Garrus and Vega) had to navigate through the top of a metal frigate that almost looked like a deep ocean oil rig floating on the surface of the ocean, all while braving the high winds, torrential rain, and falling meteor fragments. Facing off against a horde of Reapers, they had to take cover behind the angular walkways framed by jutting metal surfaces, much like those on the outer shell of the Shadow Broker's ship in Mass Effect 2.
Eventually, the team realizes that they need to jump-start certain parts of the rig, which prompts Lieutenant Cortez to dig out some universal power cells that the team must carry and deploy. This soon allows Shepard to enter a specially designed Atlas Mech that can both shoot a barrage of missiles and survive deep water dives. I wasn't able to hear much coming out of the speakers about what happens next (they were slightly muted for a reason), but the next cut-scene had Shepard taking the Atlas down to the ocean floor before it faded to black.
BioWare understandably did not share too much information beyond that. Still, they promise that Leviathan will delve deeply into history of the Reapers themselves, and that's enticing enough for most players, even if all they might get out of it is another war asset (hopefully not). Leviathan should arrive some time this Summer... so within the next two months.