When the Inter-Planetary Corporation's (IPC) crack liner, 'IPV Arcturus', took off on a routine flight to Mars, it turned out to be the beginning of an unexpected and long voyage. There had been too many reports of errors in ship's flight positions from the Check Stations and brilliant physicist Dr. Percival Stevens is aboard the Arcturus on a fact-finding mission to find out what's really happening, and hopefully save the honor of the brave pilots of the space-liner Arcturus from the desk-jockeys' in the Check Stations implications of imprecision - the nastiest insult one could cast at a ship's pilot. He and the pilots are right, it was the Check Stations that were out of position, not the ships. But that is cold consolation because before the Arcturus reaches Mars it is attacked by a small, mysterious, globe-shaped space-ship...