A teenaged girl sets out to find her kidnapped father, while a retired military negotiator returns to World to try and pick up the pieces, and prevent the use of the Protector Artifact.

Like its main character, this book seemed a little wayward, but it gave us a great view of the solar system politics and humanity's expansion, which were nearly absent in the previous two books.

There has been a transition of characters over the course of this trilogy. Ann Sikorsky and her colleagues were prominent in the Probability Moon, but had to share the spotlight in Probability Sun with a ship-tied Marbet and her captive Faller, Thomas Capelo, who figured out the physics of probability, and Kaufman, the military man in charge of the second mission to World.

In this book, Ann and Gruber have what is more like a cameo. The whole trip to World seems superfluous, as if the readers expected to see the planet once again, so the author was obliged to take us there. Essentially, the planet survived the wrecking of their society by removing the Protector Artifact and their "shared reality". Apparently it was a hard two years, but now the villages are either joining together for protection or for a power base. The one large enough to pose a threat gets conveniently blasted out of existence by the end of the section.

The only important thing that happens on World is the appearance of former porn star and former person of Great Power: Magdelena. Her son approached restricted space while drunk one day, and was blasted by a proton beam. His mother thinks he is still alive, however, and because Capelo's voice appeared on the last recording before the ship was destroyed, she thinks her son is with Capelo.

They find Capelo, who was kidnapped, on the warship near the space tunnel that opened into World's solar system. It turns out the Protector Artifact is also on that ship. In a brilliant strategy move, the Terran leader never moved the Artifact into Earth's solar system, but made everybody, including the Fallers, think he had. There were two reasons for this: one, because the first artifact blew up when it entered the tunnel (and we still don't know why by the end of the trilogy), and second, because he didn't want the artifact messing with his brain, which it was shown to do in Probability Sun. Capelo was supposed to figure out how much brain scrambling the Artifact made.

The whole plot, once again, makes a great case for physics, even to the point of implying the military chiefs didn't know the difference between theoretical and experimental physics, by throwing Capelo and the artifact together.

Capelo's kidnapping is just an excuse, however, to show us humanity's plight within the solar system. Capelo's daughter Amanda sees him kidnapped, and sets off to escape the same fate and to find him. I liked that she had been trained in science. She spent a lot of time trying to gather facts, and to be unemotional about them, even though her hormones often got the better of her.

Her first instinct was to find Marbet, not knowing that Marbet had gone to World. So she gets in way over her head, is rescued by a Catholic monk, and is ferried to Moon and then Mars by a group that opposes the government. In fact, there is a large group that opposes the current ruler, and Admiral Pierce actually manages a military coup against the current leader, Stefanak. As Amanda watches from within a monastery in Lowell City on Mars, the coup gives the society of this trilogy some depth and background. Still, it was frustrating to watch her being led around and accomplishing absolutely nothing. I often wondered what her purpose was.

When the dome of Lowell City collapsed after the coup, nobody seemed too traumatized, even though nearly the entire population of that city was killed by the event. Amanda's purpose was revealed after that: to fall in love. Konstantin was a man obsessed with Capelo, and subsequently with Amanda. I certainly hope she can change his ideas, as I find if hard to see her staying with him after the initial infatuation dies off. He was stereotypical Greek, with the masculinity complex and forwardness that accompanies it. Still, his father was very influential, and that had some possibilities for rescuing Amanda's father, especially since he was close with Pierce.

Pierce, it turns out, is crazy and delusional. The former leader was simply delusional. But he knew that he couldn't risk sending the Protector Artifact to the Faller home-system because if the Fallers had one, too, and if both artifacts were set off at setting Prime Thirteen, it would destroy the Universe. Pierce had no such reservations -he didn't believe the physics, only the results.

Marbet, Kaufman and Capelo make their way, daringly, to the solar system located one tunnel before the Faller homeworld, and manage to communicate the problem, but they failed to do so in time. So both artifacts were set off at setting thirteen -but the universe didn't collapse. Instead, the tunnels dispersed that energy, and closed forever, one by one, a very cool and unexpected result.

This is where Konstantin comes in, because Pierce is out of the solar system for a short while (conveniently). When Kaufman, Marbet and Capelo burst through the Sol tunnel, they send messages out. Thus Amanda knows her father will very likely be executed by Pierce. So Konstantin sends a message to Pierce, in the guise of his father, who has influence over Pierce, telling him of a plot against his life if he returns immediately. In this way, the tunnel to the solar system closes before the leader returns.

Marbet, however, also sent a message, conferring hero's status on Pierce, because that is the only way these three could be kept alive. I think it is inevitable that the truth would leak out; all it would take is a few well-placed slips of the tongue by any of the three, after the chaos died down. But Kaufman doesn't see it that way. He splits with Marbet by the end of the book.

As for Magdelena, she also dies, but thinking she is giving revenge on the military for killing her son. Somehow, Capelo convinces her that he can make them all suffer. We don't hear it, and because things go wrong with the Fallers, we never find out! That is also frustrating.

What I really liked about this book was the science of Probability. The author has made it into a force of nature, on par with gravity. The theory is sprinkled throughout the book, as it was developed in the last one, and it is made so probable and believable. I am very impressed by that. The theory brings the book up a notch, and overcomes some of its weaknesses, such as the aimless plot, the fourteen year old girl and her crush, and some heavy conveniences.

As the cap to a trilogy, it did its job tying things up. But the universe this story is set in is in chaos, as a new government is set up, and topples again, and the solar system is isolated once more. It is said that humanity should have cleaned up its act before heading into the stars provided by the tunnels. Now, they get their chance, by force. Who knows, maybe the tunnels are set to reopen after a certain amount of time.

The package of this trilogy is left very open-ended, as it must, and I like that. People have to live with their lives, now, and their actions. We don't get to see it, but hopefully something good will come of the tunnel closings.