Altair's abundant ultraviolet radiation, striking the top of Tenebra's atmosphere, restores the balance among the various sulphur oxides.

Throughout the day-night cycle the lakes and oceans rise and fall like tides in the Bay of Fundy, due to the heavy nightly rainfall and the easy evaporation of the liquid water (its heat of vaporization stands close to zero under Tenebran conditions).

The constant ebb and flow of hot, pressurized sulphuric acid weakens the planet's crust, resulting in frequent earthquakes and minor rearrangements of the landscape.

For two decades humans have been observing this world from the space station Vindemiatrix, which revolves about Tenebra on a synchronous orbit, one with a period just under 96 hours.

Instead of generating thrust, the rockets began to overheat and melt themselves, so the controllers jettisoned the engines and the robot completed its descent under a parachute.

A set of spines jutting from the top of the creature served as lensless eyes, making maximum use of the thin flow of photons seeping from the sky.

Trying to elude the people he is certain are tracking him, he returns to the little hilltop village of huts that he shares with his nine siblings and the robot, now called Fagin (a reference to the character in Charles Dickens' novel Oliver Twist).

Through a diplomatic error Nick gains the enmity of the chief of the people, a nine-foot brute whose name translates into English as Swift.

Fearing that Swift intends to bring his warriors to their village to murder him and his siblings, Nick dares, for the first time, to travel at night, using burning sticks (fire on Tenebra does not produce flames, but merely glows the bright yellow-orange of fresh embers) both to light his way and to fend off the raindrops that would knock him out if one enveloped him.

Raeder conceives the idea of using the craft to rescue Fagin by using the bathyscaphe's grapplers to pick up the robot and then fly it away so that Swift and his people cannot track it.

SF Impulse reviewer Tom Boardman Jr. commended the novel for its "impressive plotting and sustained suspense," noting that Clement's "consummate craftsmanship" turned a simple story into an intense one.

[2] James Nicoll, writing in 2017, noted that "(m)odern readers may be appalled by the casual way that Fagin steals eggs and raises a clutch of helpless infants to serve as his spies", and lauded Clement's choice to show that raising Tenebrans as humans, in total isolation from Tenebran culture, would have long-term negative repercussions; he also praised the egalitarian and nonsexualized portrayal of Easy Rich.