Lucky Starr and the Moons of Jupiter

by Isaac Asimov

For six years, Jupiter IX has been the site of a secret project to develop an antigravity, or Agrav, space drive; but the Council of Science learns that information from the project is released to the enemy Sirians. A month after returning from Mercury, protagonists David "Lucky" Starr and John Bigman Jones are sent to Jupiter IX to investigate, bringing a V-frog to aid the investigation.

Upon reaching IX, Starr and Bigman are warned away by the head of the project, Commander Donahue, who argues the men on the project are angry at investigation and may threaten Starr's safety. 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. During the duel, Bigman realizes Summers is sabotaging Starr's Agrav harness and forces Summers to stop at gunpoint, whereupon Starr wins the duel and gains Big Armand's friendship.

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 German Shepherd named Mutt, and has done similar favors for other workers on the project. Starr assures Norrich that he has no intention of getting Summers in trouble for the duel.

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. It is then suggested that the Sirians are using android spies throughout the Solar System.

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. The Jovian Moon lifts off on schedule, its crew of seventeen including Donahue, Panner, Summers, Norrich, Mutt, Starr, and Bigman; the latter included because Starr believes the spy is on board.

On Io, Bigman falls into an ammonial river, and is rescued by Mutt. After lift-off from Io, the Agrav drive fails, leaving the Jovian Moon falling towards Jupiter. Starr manages to land the ship on Amalthea, where they find that Red Summers is missing. An investigation reveals that Summers tricked Norrich into reporting him present on the ship while he was still on Io, whereupon Starr realizes that the Sirians had two agents planted in the Agrav project: the still-hidden robot spy, and the Earthman traitor Summers.

Panner repairs the Agrav, and the Jovian Moon returns to Io, where Starr and Bigman locate Summers. Summers admits to working for the Sirians, but kills himself before revealing the identity of the robot spy. When Norrich helps bury Summers, Starr accuses him of being the robot, and Bigman threatens to shoot him; whereupon Mutt, coming to defend Norrich, is revealed as the robot himself.

With the exposure of Mutt, Starr discerns that the Sirian spy ring on Earth must be the people who supplied Summers with Mutt, and who may be giving canid robot spies to others on Earth, and sets out to prevent them.