The Enterprise receives a message from the Sheliak, insisting they remove all humans from a planet that the Sheliak have the legal right to colonize. The Federation has no record of a colony there - there's radiation that would normally be lethal to humans - but it turns out that, about a century ago, a colony ship went off course and ended up there. They build a radiation shield, settled in, and by now have grown to a population of 15,000 or so.