"We knew the Reapers were coming, but we didn't want to accept it. Look at our planet, flames, bodies. We were not prepared for this and now we pay the ultimate price..." ~Lt. Shoultz 135th Airborne

For those unaware of the events in the game here are some possible spoilers to catch you up.

Six months after the destruction of the Alpha Relay, the Reapers finally return from dark space and begin their next galaxy-wide harvest of advanced organic civilizations. Their invasion begins in the Vular system in batarian space and rapidly spreads along the mass relay network. Within days, the Batarian Hegemony is destroyed, Earth has fallen and the Systems Alliance Navy is in full retreat, and a massive offensive is launched against the Turian Hierarchy. Over the following weeks, the galaxy's population centers are conquered or annihilated one by one as the Reapers inexorably advance. On Earth, the Reapers began destroying satellites and defunct nuclear missile silos. They soon began broadcasting orders, inviting human leaders into their superstructures to "negotiate peace." This was a ruse to indoctrinate them and pacify the population.

Commander Shepard, who had escaped Earth in the Normandy SR-2, began to actively work to unify the galaxy against the Reapers. The first step was attempting to secure a krogan-turian alliance by curing the genophage. Shepard managed to destroy a Reaper Destroyer attempting to use the Shroud tower on Tuchanka to poison its atmosphere, by luring Kalros, the mother of all thresher maws to it.

The Reapers also became involved in the geth-quarian conflict. After the quarians began assaulting the geth in a bid to reclaim the homeworld, the geth sought the aid of the Reapers, receiving upgrades and greater fighting ability at the cost of their free will. Shepard managed to defeat the Destroyer broadcasting the control signal via heavy bombardment from the Migrant Fleet, freeing the geth from Reaper control. As it lay dying, the Reaper addressed Shepard and stated that the cycle of extinction must continue, although it did not clarify further before deactivating.

Despite the victory on Rannoch, the Reapers invaded the asari homeworld, Thessia. The asari, who had been performing hit-and-run attacks against the Reapers, were forced on the defensive as the machines gradually overwhelmed them.

A small transport of civilians comes from the wreckage of the USS Kithborn orbiting Earth. The transport launched approx. 135 seconds from drove core detonation, damage caused by Reaper weaponry. Upon entering the atmosphere the transport loses power and quickly desends into the Alaskan tundra in the winter. From here your story begins...