When humans test their dimensional transporter on a human subject, it works but the subject has changed and does not want to return. So they send other human after him
You arrive in a world that is afraid of aliens. In order to contain them, they keep them chained, locked up and sold to the highest bidder to keep them as whatever they want. Science experiments, pets, girlfriends. It was up to the purchaser as long as the alien wasn't cut lose.