The invasion force is massive and the Strogg defense is substantial. After an epic battle, and thanks to the work of you and Rhino Squad, the fleet is finally able to secure an LZ for a massive command ship.
After re-grouping, you are sent again with Rhino Squad on a dangerous mission into the heart of the Strogg base. During a deadly firefight, you are captured by the Strogg and taken to their "Stroggification" facilities.
You wake up strapped to a conveyor belt that moves you slowly through the process of converting your body into one of the Strogg. In a horrific and gruesome procedure, your body is cut and fused with metal, wires, and circuitry.
As the transformation is about to be completed with the activation of your brain implant, you are rescued by a squad of fellow Marines and returned to base.
Though you were not yet "connected" to the Strogg collective, the other modifications to your body allow you to manipulate and interface with the Strogg world in ways that no human has ever been able to before.
Realizing this new advantage, the Marines mount a massive offensive to shut down "The Nexus" (the Strogg Collective), and you are the key to accessing these previously impenetrable defenses and locations.
In the end, you are the key to victory as your squad-mates disable the Strogg defensive systems, and you bring "The Nexus" offline, and ultimately destroy the Makron.