There’s no time to waste as young Council of Science member and troubleshooter David “Lucky” Starr arrives on Mercury with his tiny-but-mighty companion John Bigman Jones. No sooner do they land their ship inside the Dome city than lead engineer Scott Mindes escorts them outside onto the surface of the planet where he speaks of giant men in metal suits who can remain on the surface for hours despite the intense heat and deadly radiation from the sun. Yet each time Mindes attempts to approach them, the apparitions vanish into the shadows.

The engineer seems to be growing increasingly irritable during their discussion, until he finally pulls a blaster from his holster and fires at Lucky. Fast reflexes and low gravity save Starr’s life as Bigman tackles Mindes to the ground.

Later, Starr and Bigman are informed by Chief Medical Officer Doctor Gardoma that Mindes, a genuinely cordial young man, has been under enormous strain due to repeated acts of sabotage against Project Light, an experiment intended to produce planet-wide cooling and even disbursement of heat via orbiting space stations. Worse, Earth politician Senator Swenson has accused the Council of Science of extravagantly “wasting” taxpayers’ money by supporting Project Light. To that end, Swenson sent a ham-fisted investigator of his own named Urteil, who has managed to bully and intimidate almost everyone working on the project, especially Mindes.

Even the elderly Lance Peverale, senior astronomer of the observatory, distrusts Urteil so much that he refuses to speak of him when Starr broaches the topic.

At a banquet the following evening, tensions rise as Urteil harasses Starr and maligns the Council of Science. While Starr takes the comments in stride, the short-fused Bigman characteristically lashes out at Urteil in a violent rebuttal that begins a savage feud between them.

By way of distraction, Peverale launches into a polemic against the people of the planet Sirius, accusing them of sending saboteurs to Mercury in an attempt to thwart Project Light. Although Peverale has no tangible evidence to support his claim, the Sirians have a well-earned reputation as pirates and terrorists.

If not the Sirians, then perhaps the perpetrator is Swenson’s lackey Urteil, or someone else inside the Dome, or even the strange men in metal suits witnessed by Mindes. With as many theories as there are suspects, Starr and Bigman take to the gelid underground mines and the scorching surface of Mercury to unlock the mystery.

This is the fourth book in the Lucky Starr series and just as enjoyable as the previous three as long as you take them for what they are—fantastic, light-hearted adventures of space opera, cleverly written, but with occasional phrasing that would be considered dated and clumsy in the eyes of today’s readers. These stories are a departure from Asimov’s usual “hard SF” novels and sagas such as I, Robot, The Gods Themselves, and The Foundation Series, to name but a few.

Much like the previous volumes, the 1972 Signet edition of Lucky Starr and the Big Sun of Mercury includes a disclaimer by Asimov regarding his inaccurate description of the story’s main planet, which was based on the best astronomical data available in 1956 when the series was first published.