The Reapers invade Earth and quickly overwhelm its defenses. Commander Shepard escapes and is tasked with rallying the galaxy's civilizations to build a united military force and complete the Crucible, a Prothean superweapon that may be capable of stopping the Reapers. Old conflicts between species must be resolved before any alliance can hold. From curing the Krogan genophage to settling the Quarian-Geth war, Shepard must broker impossible alliances while Cerberus, a human supremacist organization, works to undermine the effort at every turn. The fate of the galaxy depends on the strength of the force Shepard can assemble and the choices made along the way.