When human signals begin to reach other stars it attracts the attention of some of the more advanced species in the galaxy. Most of them will observe from a distance and not attempt to land on Earth and get an up-close look at the humans and their society. The Rendor are a highly advanced species that are capable of placing their souls into human bodies and one of their number decides to do so. It is, of course, illegal under galactic law to do that, and the Rendor equivilent of policemen have to be dispatched to capture the lawbreaker and bring him to justice.
A side effect of switching a soul into a human body is that the soul already in that body is switched out into the previous body the Rendor soul inhabited. In their normal life the Rendor do not use bodies, so the soul of the first body they switch to has nowhere to go and thereby leaves the mortal universe and the soul of the last body they inhabit will have no replacement and thereby becomes vacant. Because of this, and that fact that most Rendor are relatively moral, they are generally reluctant to do that. When they must, they generally use a body form closest to that of the displaced soul and attempt to keep it unconscious while they borrow the body of the other species. Their criminals have no such scruples and often leave quite a wake behind them.