Thirty years ago the Empire of humans found, investigated and decided to use a blockade of space ships to prevent the spread of an alien culture called the Moties, who inhabited a single star system and were only just discovering faster-than-light travel. Now, because of a change in the stellar neighbourhood, the Moties will be able to leave their home system and spread across the galaxy, which will be bad for mankind because they combine great ingenuity with rapid and unstoppable population increase. Horace Bury, a wealthy trader, and his pilot Kevin renner (who is actually a serving member of the Imperial Space Navy) have spent the last thirty years travelling the Empire checking that no Moties have escaped; now they return to the Motie system to try to prevent the expected breakout. Their only real weapon is a symbiotic worm which can stop the continuous cycle of population increase that has caused so many problems. As soon as they enter the Motie system they are taken prisoner. Their plans have to be revised to find a solution that will be acceptable to both the Empire and the Moties.