Aliens were mass controlling every human being by jumping on their back and plugging in to the neck. It looked like this:
I can't remember anything but one scene, in which US soldiers were in a big hangar with their equipment, and had a video chat with the president. The president asked them to prove they were not being controlled by aliens, and a general said "No problem," started unbuttoning his shirt, and then turned around and the video signal was lost. (They cut off the video on purpose, because they actually were controlled.)