 This is a familiar variation on the alien invasion theme.In the infiltration scenario, the invaders will typically take human form and can move freely throughout human society, even to the point of taking control of command positions.The purpose of this may either be to take over the entire world through infiltration ("Invasion of the Body Snatchers"), or as advanced scouts meant to "soften up" Earth in preparation for a full-scale invasion by the aliens' conventional military ("First Wave").