While tensions simmer between Earth and its colonies on Mars, Venus, and some of Saturn’s moons (collectively known as the Federation), Earth intelligence agent Bertram Sadler travels to the moon observatory in search of a spy leaking information to the Federation.

Working undercover as a cost accountant performing a financial audit of the observatory, Sadler gains access to all departments and staff members—who at first greet him with suspicion. Over time, Sadler builds a list of top suspects while both the Earth and the Federation create weapons of mass destruction in a prelude to war.

The first half of Earthlight is slow and plodding as Sadler meets various members of the observatory’s staff and is schooled on various as aspects of their operations and of astronomy. The only two interesting plot points are the unannounced landing of government ships in an area of the moon normally off-limits, and the two astronomers who decide to venture out in a vehicle to investigate.

The tension in the story begins to build in the second half when the observatory receives a communication warning the staff to dismantle critical equipment and take shelter underground. A war is coming, one that will decide who has control of the moon’s abundant supply of heavy metals deep within its core.