For a century the colonists had tried to make a living on Mars and, more importantly, to unlock the secrets of the abandoned cities that they found on the planet and now some people just wanted to give up.

Nelson assumes that the burglar was looking for the letter that Leroy Perrault, a scientist with the Interplanetary Bureau, had given to him to pass on to his father, John Carson Parr.

After arriving on Mars Nelson learns from his father that the planet is indeed being abandoned and over the next few days the remaining three hundred colonists are put on ships and sent to Earth.

Perrault’s letter has authorized this clandestine group because certain anomalous events noticed over the previous century indicate that the Martians are still present, hiding in their impenetrably sealed cities.

To catch out the hidden Martians the six men ride a small rocketship to Phobos, the larger and innermore of Mars’s moons, and set up camp with their telescopes.

To keep a longer watch on the crates Nelson and Jim take their rocketship’s lifeboat to Deimos and set up the spare telescope some distance from their landing site.

Trying to spot the murderers while at the same time evading them, Nelson crosses the surface of Deimos in long, gliding bounds, hoping, in some desperation, to find the killers’ spaceship and somehow take it.

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.

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.

The book was reviewed by “A jaunty story, somewhat more humorous and lighthearted than the others in this series, takes young Nelson Parr, a Terrestrian born in the Martian outpost and research center, through a set of adventures that unlocks Mars’ secret.