The 2nd adventure in this new series. Two experimental robots have vanished while undergoing field tests on the surface of the Moon. All evidence points to the presence of an unknown person in the area, but no one can reasonably account for how the person could have been there. Sandra Swift decides to investigate and, accompanied by Bud Barclay, travels to the Swift Enterprises Moonbase. From there they go to the last reported position of the robots, but suddenly end up falling through a narrow crevice. Now trapped deep beneath the surface of the Moon, Sandy and Bud struggle for survival, exploring a vast system of underground caverns as they attempt to contact help. But they soon encounter additional mysteries in the form of a lost human colony living in a ruined spaceship, as well as an insane woman bent on revenge. Fighting to stay alive, Sandy and Bud join forces with the lost colony, squaring off against an adversary who will stop at nothing . . . including nuclear annihilation . . . to kill anyone who stands in her way.