Iain [M.] Banks - Matter-Unabridged

The book follows the experiences of three members of the royal household of the Sarl, a feudal, pre-industrial humanoid race living on the eighth and ninth levels of the Shellworld of Sursamen. Shellworlds are ancient artificial planets consisting of fourteen nested concentric spheres internally lit by tiny thermonuclear "stars," whose layers are inhabited by various races. Constructed for an unknown purpose by a long-dead race called the Veil, Shellworlds are guarded and mentored by progressively more advanced species, up to the level of what the Sarl call "Optimae." [The Optimae themselves, particularly the Culture, prefer the term "Involved."> Approximately 4,000 Shellworlds were known to have existed at one point, but almost 2,000 have been deliberately destroyed by another later, vanished race, the Iln. Like many Shellwords, the hollow core of Sursamen is known to be inhabited by a mysterious creature called a Xinthian Tensile Aeranothaur, whom the Sarl worship as their "WorldGod."

Prince Ferbin, the heir to the Sarl throne, has to flee his home level on the Shellworld and the Shellworld itself after witnessing the murder of his father, King Hausk, by Mertis tyl Loesp, the King's second-in-command. Prince Oramen, Ferbin's studious younger brother, is unaware of the treachery and trusts tyl Loesp fully. After Ferbin's disappearance—and presumed death—tyl Loesp takes on the role of regent, ostensibly until Prince Regent Oramen comes of age and can be crowned king.

The Oct, the direct mentoring species over the human Sarl, meanwhile have been organizing the takeover of the ninth level of Sursamen, using the Sarl royal court and military as their pawns. It becomes increasingly clear that they are searching for something hidden in the Nameless City, a metropolis buried under several hundred million years of sediment which is currently being stripped away by the giant Hyeng-zhar waterfalls. The ninth level was only recently re-colonized in a move by the Oct, which was retrospectively validated, with reluctance, by Sursamen's mentoring races, the Nariscene and the more senior Morthanveld. (The Culture has no jurisdiction over Sursamen.)

Elsewhere, Djan Seriy Anaplian, another child of King Hausk, had left Sursamen fifteen years earlier to become a member of the Culture as part of a cultural exchange and later joined the Culture's covert Special Circumstances (SC) intelligence and intervention organization. Anaplian decides to return to Sursamen, originally to pay her respects to her dead father and presumed-dead brother. (Initial reports from Sursamen only described the king and prince's causes of death as combat related.) Meanwhile, the fleeing Ferbin is seeking out his long-absent sister hoping that, with her powerful connections and SC access, she could help install him as rightful king of the Sarl.

Within the Culture, other channels of intelligence later validate suspicions that the Oct are planning something mysterious on Sursamen and SC contacts Anaplian en route via a human-like avatoid, Klatsli Quike. SC asks Anaplian to investigate, "upgrading" her bereavement travel to an official Culture mission. Quike, a virtual representative of the undercover SC ship Liveware Problem, is soon replaced by another LP avatoid, Pone Hippinse, as Anaplian's tactical partner in the mission. Her rather irritable combat drone (Turminder Xuss) had stowed away in her belongings at the beginning of her journey, disguised initially as a Knife Missile resembling a dildo. Anaplian has had most SC tactical enhancements to her mind and body disabled for her personal journey out of diplomatic considerations, but she restores most of them, and Xuss will prove critically useful. At this point, Anaplian is reunited with her fugitive brother, Ferbin, and his faithful (but increasingly independently-minded) servant Choubris Holse.

Arriving at Sursamen on board the Liveware Problem, the team (Anaplian, Hippinse, Ferbin, Holse and Xuss) successfully infiltrate their way down to the Shellworld's ninth level to confront their adversary, tyl Loesp, and protect their brother, Prince Oramen, both of whom are overseeing the excavation of the Nameless City. By the time the team arrives, Oramen has survived assassination attempts by tyl Loesp, but has been killed by a mysterious and powerful entity long entombed in the ancient ruins. Oramen had discovered—too late—that the entity is a ruthless machine created by a long-dead race known as the Iln. In the distant past, the Iln were responsible for the destruction of thousands of shellworlds, for reasons unknown, before their own presumed extinction.

The liberated Iln machine's immediate intention is the destruction of Sursamen and possibly other Shellworlds. Its nature is a horrible surprise to the Sarl humans and especially the Oct, who thought that they were excavating one of the "Involucra" (aka the Veil) who had originally built the shellworlds and from whom they claim to be descended. The Iln entity kills all nearby Sarl and Octs with a small thermonuclear explosion—after thanking them for their help—and escapes towards the core of the Shellworld, planning to kill the Xinthian "WorldGod" and then destroy Sursamen by generating and detonating antimatter.

With both their adversary and their brother dead on the ninth level, Anaplian and Ferbin descend towards the Shellworld's core level with the rest of the team, equipped with intelligent SC-technology combat suits. The Liveware Problem itself speeds down a vast shaft into the Shellworld's interior to join the team, but sustains substantial damage from Nariscene weapons during its descent to the core. The team is thus outgunned by the Iln, who has taken over the programming of a Morthanveld guard ship and twelve other drones secretly emplaced in the core. Xuss is MIA. Hippinse's parent ship, the Liveware Problem, disposes of all but two of the Morthanveld drones then sacrifices itself in a suicide attack on the hijacked Morthanveld guard ship. Hippinse himself is killed eliminating one of the remaining drones; Anaplian accounts for the last one, but the Iln remains too strong for the remaining compatriots, whose resources are seriously depleted.

In the end, the self-sacrifice of two (Ferbin and Anaplian) saves one (Holse) and defeats the Iln machine, thus saving the entire Shellworld. Ferbin exposes himself to the Iln machine and is killed outright, but satisfies the enemy that the humans are not harmful when disarmed and permits Anaplian a much closer approach. Despite her advanced armor, she is shredded beyond the point where a non-SC enhanced human would be dead, but she remains conscious enough to detonate the tiny grain of antimatter in her skull that provides power to her SC-enhanced body. At this point, all presumed heirs to the Sarl throne are dead and Sursamen's near destruction will lead to extensive shifts in political power there, including Sarl's interaction with its mentor races.

In the epilogue, Holse, the lone survivor of the Iln encounter, rejoins his family after a long absence, accompanied by the surviving Liveware Problem avatoid, Quike. He declares his intention to become a political leader of the Sarl, with the secret backing of the Culture. From Holse's survival, the Culture could be presumed to have a reasonably complete picture of events; from the continued existence of the Shellworld, one may infer that Anaplian's sacrifice successfully foiled the Iln.