After the Sirians make a case for their base on Titan, Lucky enters the planetary system against their express permission.

I quite enjoyed this book, and I loved the main character's plan for turning back the invasion, but there was one point I completely disagreed with, I'm tired of Bigman's fighting, and as with the other novels in this series, there is too much exposition that could have easily been integrated into the story.

The last book in the series actually brings some development to the relationship between Earth and the Spacer planets, led by Aurora, which is not yet known by that name. Instead, the villains are called Sirians (as they have been throughout).

The Sirians, who use robots like all spacer worlds do, have set up a colony on Saturn's moon Titan. Lucky thinks this puts Earth and the Council of Science in a bad light, because they didn't notice it at all, not to mention that is is against the normal convention of colonization. In fact, there is so much traffic in the Saturnian system that it could be the prelude to an invasion.

The book starts with Lucky, Bigman and Councilman Ben Wessilewsky on their way to Saturn. They followed a Terran spy out all the way to Saturn, and after his craft exploded, they were ejected from the planetary system by the Sirians.

Lucky and the two others return, sending a message to the Council back on Earth. The suspense is built up by keeping Bigman in the dark and having him rant about it, while Lucky keeps his mouth shut, and even when the story takes place from his point of view, he doesn't reveal much.

Once again, I don't like the way Asimov stops the story dead to give us scientific facts about Saturn. There is, at least, some humor in it, in that Wessilewsky lectures to Bigman, who hates being lectured! The bantering between those two is more cartoonish than anything else, which I think it what the author was after, but which I didn't care for much at all. Bigman is always telling Ben that he'll pound his face, and so on, while Lucky always stops the fight from occurring. But all the science could have been delivered more naturally.

They try to arrive stealthily, from a polar orbit, but to no avail. The robot-piloted ships spot them and give chase, but not before Lucky goes outside the spaceship (where we learn much later he captures the message capsule the spy ejected before destructing). His ship is using the top-secret agrav technology that allows him to easily pilot within the high gravity of the gas giants, so he self-destructs them before being captured.

Before that, though, he makes a high-speed pass through the Cassini Division of the rings, and plows straight into Mimas, a moon purported to be made entirely of snow and ice. The ship thus escapes the robot sentries, burrowing into the moon with its overheated emitter. They leave Wessilewsky in an air bubble created by the ship's heat. At this point, I had no idea why this was done. When the reason was revealed, however, I thought it was brilliant!

So Lucky and Bigman are captured and brought to the humans on the Titan base. Lucky (and later other visitors) think it is absolutely beautiful, so it's unfortunate that it is an invasion of the Sol solar system. The leader of the colony is a man who dislikes Earth-men considerably, and Bigman in particular. As epitomized on Solaria in The Naked Sun, spacers prize beauty, and Bigman is anything but beautiful. They actually spend much time discussing the differences between Spacers and Terran points of view, where it relates to beauty.

So it's hard to believe that Devoure, no matter his disagreements, agrees to get into a fistfight with Bigman. Bigman was to be sent on a spaceship with no resources, to die a lonely and painful death unless Lucky testified to his own illegal actions, but he managed to convince his robot captors that he would kill himself, triggering a First Law conflict. He even destroyed a robot.

Even though Devoure is not liked at all by his colleagues, and is a very disagreeable person, they don't mutiny against him. One turns his back on the fight, but the other goes along with it, showing that even spacers have people who will follow authority when it is so obviously wrong.

Bigman wins the fistfight, of course (as Devoure apparently never hit anybody in his life). But then came the sequence that nearly ruined the book for me. Devoure manages to successfully convince the robots that Bigman is not human. Even if he is a Spacer, and can talk to robots in a way that Martians couldn't, there is no way at this stage of galactic evolution that robots could be remotely convinced that a human wasn't human, by words alone. They have their own perceptions, they know that humans lie, and they would undoubtedly require hard evidence before acting on such a claim. It took millennia before the Solarians managed such a feat in Robots and Empire, and even then, Daneel, Giskard and Gladia didn't believe it.

But then Lucky saved Bigman, first by calling out an alarm on the colony emergency frequency, then by trading Bigman's life for the much bigger prize of Councilman Wessilewsky. The Sirians jump at the chance, and extract the Councilman from Mimas. We knew that Lucky wasn't about to betray his honor, though everyone else thought so.

The conference went about as expected. The Sirians (and everyone else) thought they had the upper hand, and would win. Earthmen were protesting in the streets, the Council of Science was about to be disbanded, Lucky branded a traitor, and so on... Everything seems to go the Sirians' way. They correctly claim that Titan was uninhabited, and that Earth had no interest there. But then Lucky testifies, and the simplest concept allows him to snatch victory from the Sirians.

With Asimov, it is more often than not the simple concepts that prove to be the key. This was the case in so many of the Robot novels, and especially the Foundation novels. Here, Lucky reveals that he did enter the Saturnian system against requests from the people who already settled there, but he did so to colonize Mimas! He successfully proved that the Sirians were applying one standard to themselves, and another to him and Wessilewsky (who colonized the small moon)! This frightened the Spacers enough to vote on Earth's side, for they didn't want anybody colonizing any small moon or planet in one of their solar systems.

As with any mystery, I think the big reveal is the most important part of the book, and this one was terrific. It offset Bigman's behavior and even the behavior of the robots. So the series ended with a good plot, an interesting story, and a major development for Earth's relationship with the Spacers. I can't ask for much more than that.