A Matter of Logistics

After being banished into deep space and forgotten, these outcasts made contact with alien species and raised a great army for their return. But an ensuing stalemate between the forces required new tactics. So the enemy infiltrated the isolated planet as stealthily as a virus and like a virus was mutating the native beings for the enemy's use. And they recruited a penetration agent of peculiarly powerful mental abilities, built him a perfect clone body, and sent him to find out if the infection was so advanced they might have to kill the host by destroying the planet that its inhabitants called Earth ... It was simply a matter of logistics!