You have now become the President of the United States, so watch the cutscene at the beginning of this mission. Walk forward when you get control, where you'll be stopped by Keith David, who will ask you to choose between curing cancer or stopping world hunger. Make your choice (it doesn't really matter) and you'll be stopped two more times, where you have to make a choice.
Watch the cutscene of the press conference, when all of a sudden, an alien invasion occurs! Once you gain control again, press the button to knock the Secret Service off of you, then start going down the hallway, shooting the Zin in the way. You'll meet up with Asha, who says to follow her, but go into the Oval Office first to upgrade your weapon to something with a little more oomph.
Exit the office and start killing the Zin aliens while your Cabinet gets abducted. Once you make it outside, you'll hop into a giant cannon. One button fires the cannons, while another will fire some rockets. Keep shooting down the enemy ships that are marked until a battleship comes, which isn't that much more difficult to destroy.
Unfortunately, the battleship crashes into the cannon, but you climb out and make your own entrance into the alien ship. A scripted fight with Zinyak will ensue, complete with QTEs, but it ends with you being defeated and captured by Zinyak.
