The star ship Lief Erikson, a construction ship, must stop in an unexplored system to conduct major repairs.
They discover the system to have a large human population, they assume it to be a dark colony. This colony knows no earth language and lacks technology freely available when the first ships left earth.
After communication is established a meeting is set up. Instead the locals raid the Erikson, they kill or kidnap dozens of people.
In the center of this is Roger Powell, a new citizen of the Erikson. Originally hired as a painter he is trained as pilot, during the raid he is piloting the one improvised warship the Erikson has. He becomes a hero, and against his wishes is made a leader.
In the aftermath they discover the humans in the system are not human in the area that matters most. They are forced to fight a war if they ever want to leave.