Saturn’s largest moon, Titan, has been claimed by invaders from the planet Sirius, the first of many extrasolar Earth colonies. Over several generations, the Sirians and their allies on many of the outer worlds turned against their planet of origin, citing social, scientific, and military superiority after generations of ethnic cleansing. Despite an intergalactic law stating that any planet in an inhabited solar system belongs to the people of that system, the Sirians have constructed a military base on Titan as their first step to attacking Earth. The Council of Science, an organization sworn to protect Earth and its neighboring planets with minimal violence, fears that the Sirians have become too powerful to defeat.
After a Sirian spy named Dorrance escapes Earth custody, Councilmen David “Lucky” Starr and and his tiny-but-mighty companion John Bigman Jones set off after him in their ship, the Shooting Starr along with several vessels from the Terrestial fleet. They pursue Dorrance into Saturn’s rings, where his vessel is destroyed. However, a Sirian vessel contacts the Shooting Starr and orders it away from Saturn, informing him that the Sirians now occupy Titan and any aggression from Earth will be considered an act of war. Starr retreats and orders the Terrestial fleet to do the same.
Later, Starr, Bigman, and fellow councilman Ben Wessilewsky return to Saturn in an unauthorized expedition aboard the Shooting Starr to find a information capsule that Dorrance had stolen from Earth. When Sirian ships again detect their ship and pursue, Starr “crashes” the Shooting Starr on Mimas, Saturn’s closest moon. There, he leaves Wessilewsky behind and takes off again with Bigman–only to be captured by Sirian forces. The leader of the Sirian base on Titan, an irascible tyrant named Devoure, attempts to coerce Starr into confessing to espionage and to testify against Earth at an upcoming peace conference on the asteroid Vesta. Devoure offers to spare Bigman’s life in exchange for Starr’s compliance.
Will Lucky Starr betray Earth at the conference and join the Sirians? What of Councilman Wessilewsky on Mimas? Will the other planets vote against Earth and allow the Sirians to occupy Titan as a prelude to war?
Lucky Starr and the Rings of Saturn is the final volume in a series of six. Much like its predecessor, Moons of Jupiter (reviewed here), Rings of Saturn takes on a noticeably darker tone than the first four books. 
I was forced to wonder if perhaps Asimov started off with the intention of creating a light-hearted space adventure, but later allowed real world tensions of the time, such those between USA and the USSR, to inform his fiction. The tension and stakes in Rings of Saturn are higher than they’d been in the previous books, but it could also be construed that each story builds upon the last to culminate in this final confrontation between Earth and Sirius. Though it’s easy to see the potential for future adventures in this universe.
its four predecessors. This was the first time in the series that David Starr did not always have the upper hand in every predicament and was, in fact, foiled on multiple occasions both by his own incorrect assumptions—or inexperience—and by the ingenuity of the Sirians. Of course, Bigman’s typical immature and rash antics did little to help the situation, except for a tense anti-gravity brawl at the beginning of the book.