Alien sociologists on a field trip.
Aliens come to Earth looking to study humans. Their ethics prohibit them from forcing humans to come with them (or tricking humans into it with false promises, etc). However, they telepathically find a few thousands people who would be willing to simply volunteer to leave Earth behind and come with them.
One of the humans suggests pulling a hilarious prank: before volunteers leave, aliens announce that all humanity will be forced to take a classic private red/blue button vote. Humanity has a week to coordinate. During the actual vote aliens aren't even keeping track of the votes. After the votes are cast only the people who originally volunteered to go with the aliens are replaced with very convincingly looking dead bodies (of themselves) while the volunteers themselves are safely teleported away. Aliens depart. For everyone else on earth it looks like red button won the vote and the volunteers were the only ones who actually voted blue. Also all volunteers were telepathically prevented from leaking the information about this ongoing prank and their planned departure while they stayed on Earth and aliens don't leak that information either.
Share your ideas what will happen back on Earth after aliens depart.