The Lone Sentinel is set in an unspecified future after "the last war" on Earth, when earthlings have settled the planet Azure and attempted to recreate conditions on Earth, with which they have lost all contact and which is probably extinct. Endangered by the hostile conditions on Azure, for which they have been ill prepared, the earthlings owe their survival to the Helgatites, space aliens whom they repay by cultivating biosote (a coral-like substance needed by the Helgatites, but in short supply on their planet). Now, several generations later, a Helgatite spaceship continues to orbit Azure to protect the planet, descending only at ten-year intervals to harvest the biosote. Thus, although Azure's inhabitants know there is intelligent life elsewhere in the universe, their's is a "closed" planet; their only contacts are with the Helgatites, and then only through Trust Control, the somewhat authoritarian governing body established by the...