Lucky is sent to Venus to investigate a threat by apparently mind-controlling beings intent on ruling the people who live there.

Finally, we get a true mystery story in the style of Isaac Asimov, in this series. While the last two books meandered around and didn't even tell much of a story, this one had a real purpose, and Lucky Starr's investigative powers were shown here. The mystery would even be solvable by somebody who studies mystery books!

The real star of this book is Lucky Starr, of course. His sidekick Bigman isn't really worth having around, except that he can act dumbfounded and be disparaged by Lucky when the sleuth uncovers various clues. As is typical with Asimov's books, although the worlds that his characters populate are civilized and enlightened, when cornered, people still respond in passionate violence, even though several witnesses are present, and it obviously will not change the outcome.

The premise of the book is that various people on Venus are falling into hypnotic trances, and an agent of the Council of Science is believed to be attempting to steal proprietary knowledge about the yeast that Venusians produce. Lucky is sent to investigate.

Venus here is completely wrong, from a scientific standpoint. Asimov mentions in his introduction that we have to imagine that it is a mythological planet, instead. It is actually a wonderfully-realized planet, full of varied life, an ecosystem that is unique and makes sense, and a human sense of ingenuity in order to live there. What I find difficult to accept is the various statements in the book that tell us nobody knew what Venus was like until humans arrived. When these books were written, of course, there were no satellites. But even Asimov should have foreseen the advent of robotic spacecraft, probes that would look onto the surface of the planets well before people got there.

Fortunately, that doesn't really detract much from the good story we get here. Lucky's classmate Evans is imprisoned on Venus. I find it strange that Evans and Lucky could have gone to school together, while Evans is on the Council, and Lucky is repeatedly stated to be the youngest Council member by far, and likely will be for years to come. Evans was evidently much older than Lucky when they were roommates. Regardless, Evans has no excuse for his actions as he stole the yeast formula, but Lucky knows he must be innocent.

A sudden crisis at an airlock to the floating city, which is protected in sections by forcefield-infused steel, causes a portion of the city to be evacuated, and all security members to be tied up. Bigman manages to help diffuse the situation, but it is Lucky who realizes that the crisis is a ploy to break Evans out of prison so he can escape the city. But he arrives too late, so he and Bigman chase after Evans in the deep ocean. They have been lured into a trap, however, and are stuck underneath a giant ray-like fish, kilometers wide and shaped like a bowl. It is being used to fire its water-jet at the submersible and kill the occupants. But Lucky goes out into the ocean in his pressure-suit and kills the thing, so they can escape. The sections in the ocean were brilliantly described and quite exciting. Unlike the other stories so far in this series, I didn't once think the situation was unbelievable.

The key to the mystery is the species called the v-frogs. They have a telepathic link that can be used to take over human minds. Evans is taken over by the v-frogs and forces Lucky to return to the city with his blaster, but Bigman causes a distraction, so they can rise to the surface. Lucky is unable to send a message to Earth via the space stations, though, because the v-frogs take control of his motor functions. Within the submersible, he has a power struggle over control of his body, but the v-frogs tip their hand, saying strange things that they could have no idea about -like fire, showing that somebody else was controlling them!

I think the portrayal of the v-frogs was a little simplistic in what follows. Apparently they have such an addiction to anything with hydrocarbons in them that they are instantly distracted. So Lucky finds some petroleum jelly in the first-aid kit and flushes it outside, which breaks their hold on him, and he sends the message out. Then, as they approach the city, its weapons are turned on their submersible. But Lucky announces over the intercom that they are bringing a hydrocarbon fuel shipment on board, which saves them.

Then, in typical Asimov style, Lucky baits the trap. He knows that the v-frogs are not acting of their own accord, but through somebody else, and he is sure he knows who. Apparently knowing that the chief engineer has a meeting with the mayor (a councilman himself), Lucky schedules an emergency meeting with the mayor also. There, he accuses the mayor of treason, getting the man so mad he attacks Lucky physically. This breaks the engineer's concentration, so Bigman can shoot his computer. I liked the portable computer, which Lucky watches with awe, and which was supposed to be believable at a time when nobody could envision a laptop or palm-sized computer! Now it is a hilarious curiosity. But it is the key to the entire mystery, as the engineer was controlling the v-frogs through it, probably with grand plans to rule the galaxy. What gave him away? His love for his new wife, whom he didn't even warn about the impending disaster if the airlock was opened during that crisis!

I didn't think the denouement was necessary, or maybe it should have been different, because otherwise I would have complained about the lack of one: That the engineer could be convinced to build another computer such as that, for the good of mankind, to cure diseases and so on, seems rather unlikely, even in this fantasy world. What motivation would the man have, unless he was content to build such wonders as laptop computers -instead of grand ambitions to rule mankind?

Regardless, the book was very interesting, and extremely well written, reminding me of why I enjoy the earlier Asimov novels as much as I do. There was still a little too much pompousness in Lucky and the Councilmen, but much less than in the previous two books. And Lucky didn't even use his magic mask! But the mystery was well-built, and though I dreaded the use of mind-control the moment it was mentioned (an overused Asimov plot device), in this case, it turned out to be an alien lifeform, which was unforeseen and pleasantly surprising.