The first half of the novel takes place on what was then the outermost known satellite, Jupiter IX, discovered by Seth Barnes Nicholson in 1914.

Starr and Bigman land nevertheless, and are met by a large group of workers led by a man named Red Summers, who insists that Starr take part in a duel in an Agrav corridor against a much larger man called Big Armand.

At their quarters, Starr and Bigman are met by their neighbor, a blind man named Harry Norrich, who tells Starr that Summers is a convict on Earth, but has earned responsibility on the project, and is hostile to Starr for fear of having his past crimes revealed; whereas when Norrich's seeing-eye dog died, Summers obtained another, a German Shepherd named Mutt, and has done similar favors for other workers on the project.

The next morning someone kills the V-frog while both men are distracted; whereafter Starr and Bigman tell Donahue and James Panner, the chief engineer on the project, that because the V-frog telepathically induced affection in all whom it met, only a robot, immune to emotion, could have killed it.

Donahue, unconvinced, orders the launch of an Agrav ship, the Jovian Moon, to Io the following evening, and forbids Starr to conduct his investigation until after it returns.

After lift-off from Io, the Agrav drive fails, leaving the Jovian Moon falling towards Jupiter.

Panner repairs the Agrav, and the Jovian Moon returns to Io, where Starr and Bigman locate Summers.

Galaxy's Floyd C. Gale reviewed the novel favorably, calling it the best of the Lucky Starr series and saying that Asimov "milks the situation for all its thrills, while painlessly imparting scientific lore and a code of ethics.