A consortium of spacefaring races, including humans, recruits Mesklinites, the centipede-like natives of the high-gravity planet Mesklin, to explore Dhrawn. The recruits include Barlennan and Dondragmer, respectively the Captain and First Mate of the Bree, a merchant vessel of Mesklin, which sailed to Mesklin's south pole to rescue a probe sent by humans. Now, thanks to institutes of learning set up on Mesklin, the natives have produced capable explorers who can go where other races cannot.
Barlennan is in command of the main base on Dhrawn while Dondragmer commands a "land cruiser", the Kwembly. This is a tracked vehicle about 30 meters long, 6 meters high and the same wide. It is designed to move like a large worm on independently steerable trucks. The power is supplied by self-contained fusion generators but the controls are simple pulley-and-rope systems using Mesklinite materials which the crew can repair themselves. There are several more cruisers, and each has audio/video links for communication with satellites.
The humans and others are on a satellite in synchronous orbit above the explorers on the ground. Unfortunately the planet's slow rotation means that they are about 10 million kilometers above the surface, and signals take over 30 seconds to travel to the satellite. Real time conversations are therefore impossible.
On the satellite are linguist "Easy" Hoffman and her son Benj, who is both an engineer and a linguist. Both speak fluent Stennish, the Mesklinite language, and have formed close personal relationships with the explorers on the surface. They are later joined on the satellite by Ib Hoffman, Easy's husband and Benj's father.
One of the cruisers, the Esket has apparently suffered a catastrophe and all the crew have vanished, much to Easy's dismay. The cruisers communicators still function, but all they show are views of a deserted ship.
In reality, Barlennan is executing a complicated deception. A wily negotiator who successfully blackmailed the humans into giving him technology in Mission of Gravity, he is apparently plotting to gain yet another advantage from the situation he created. The exact nature of his objectives is unclear.
Soon, however, the Kwembly is in very real trouble. As the planet warms, the complicated phase changes of water and ammonia mixures at these low temperatures mean that a frozen lake can melt in seconds, carry the ship off in a flood, and equally suddenly leave it hung up on large rocks, unable to move as the liquid around it freezes again, trapping some of the crew below the surface in their protective suits.
These events, along with some bad timing, lead to the discovery of Barlennan's trickery when Easy recognizes a friend of hers from the Esket as he appears, as if from nowhere, but actually from one of the dirigibles that Barlennan is clandestinely using to move his men and materiel around.
The Hoffmans would prefer to deal honestly with the Mesklinites, but they have to deal with the prejudices, not only of fellow humans with political motives, but with the more paranoid of the non-human supervisors of the mission. Having an inkling of what Barlennan is really after, Ib Hoffman finally breaks the deadlock with an idea that will save the crew of the Kwembly and make the mission successful beyond the dreams of its planners.
To some extent, the narrative is much weakened by the focus on the science. There are long passages of exposition intended to show both what can go wrong in a seemingly well-understood situation, and how individual ingenuity and perseverance can overcome problems. The human characters seem made-up and lacking in depth, simply being people with extraordinary abilities trying to deal with a situation where they are getting information that is late and, they later realize, unreliable.
The final lesson in the novel is that full and complete disclosure is the only way to succeed in a situation where unpredictable changes can occur. Ib Hoffman relates the story of how his son Benj, at the age of thirteen, designed and built SCUBA equipment, carefully calculating all the parameters, only to come close to dying on his first dive because he lacked a crucial piece of information about the physiology of breathing, one which all his knowledge of physics and engineering could not have predicted.