Marauders, an immense horde of interstellar pirates in a vast armada of huge, black ships, were headed their way. They responded by building two giant, spherical spaceships, loading their entire population into them, and fleeing into interstellar space for a three thousand-year journey to Sol, where they arrived and went into orbit around Mars sometime in the 1600s. Suddenly realizing that the Phobosian spaceship must still be available, Nelson coerces Kunosh into taking him to it. They arrive in the hangar just in time to see the five Phobosians that Nelson helped capture boarding the craft and leaving Deimos in it. Returning to the Deimos control room, Nelson is put into television contact with the Phobos control room and the leaders there. The Phobosian leader demands that the Deimosians capture Nelson so that his men can pick him up and bring him to Phobos, where the Deimosians cannot secretly murder him as they did Jim Worden. Before the Deimosians can act panic breaks out – the lead elements of the Marauder fleet have been detected crossing the orbit of Pluto. In the panic Nelson drags Kunosh back to the spaceship hangar and compels him to explain the controls of a cube-shaped vehicle. Once alone he lifts the ship off Deimos, which has thrust itself out of its orbit to head into interstellar space, where it will never be found. Phobos has also left its orbit and is headed toward Earth. Trying to keep up, Nelson is captured by the Marauders. It turns out that the Marauders are the Martians, descendants of people taken from Earth to Mars half a million years ago by unknown visitors from the stars. Having learned the technology and techniques of interstellar flight, they tell Nelson, they pack up their entire population every ten generations or so, seal their cities, and take off on a millennia-long interstellar road trip. They have just returned from their latest adventure and find that they must now prevent a war with Earth. A single fast cruiser takes Nelson to Earth, lands at the Capitol, and Nelson and the Martian representatives convince Earth authorities that peace is best. Publication history 1955, USA, John C. Winston Co., Hardback (xi + 206 pp) 1955, Denmark, Winthers Forlag, Rumfart-Serien #7, Paperback, as Mysteriet på Mars (The Mystery on Mars) 1957, Germany, Erich Pabel Verlag (Rastatt, Germany), Utopia-Kriminal #21, Paperback, as Das Marsrätsel (The Mars