The Hydrogen Sonata is the final novel of The Culture series. Set approximately one thousand years after Consider Phlebas, the book is a Cloak & Dagger thriller set against the backdrop of the end of a powerful, venerable civilization. The Gzilt, a militaristic civilization of reptilian humanoids, have decided to Sublime by referendum, choosing to leave the physical world and Ascend to a Higher Plane of Existence. As scavenger civilizations circle to pick over the eventual remains, a secret going back to the dawn of the Gzilt civilization threatens to derail the whole process and spark a war if revealed. Gzilt citizen Vyr Cossont finds herself caught in the middle of the Culture frantically trying to find the truth as well as put out the fires, and factions of the Gzilt who wish to protect this secret at all costs.
The Hydrogen Sonata provides examples of:
- Apocalypse How: A Culture Mind performs some insane hyperspace acrobatics that could have, it notes, destroyed a densely populated planet had even the slightest mistake occurred.
- Ascend to a Higher Plane of Existence: The process of Subliming is finally described in some detail, and serves as the major plot event that sets the story going.
- The Bad Guy Wins: Although they fail to kill the main heroine, virtually all the "bad guy" factions get pretty much everything they want by the end, up to and including ascending to a higher plane of existence- the nearest thing the Culture universe gets to actually going to Heaven. This even includes the main bad guy, who has personally killed hundreds of innocent people. Meanwhile, their victims are left just plain dead. The heroine is so disgusted by it all that she refuses to ascend, and is left nearly alone (albeit by her own choice) on the mostly empty remains of her now-dead civilization, whose ruins are soon to be occupied by the more warlike of the scavenger factions.
- Cool Starship: As a Culture novel, this is there in spades, but a special mention must be made of the eccentric Culture ship of indeterminate class Mistake Not..., whose excessively long full name is an inside joke among Culture Minds. No one is really certain about its combat power (even itself), but its wiles demonstrate that it lives up to its full name: Mistake Not My Current State of Joshing Gentle Peevishness for the Awesome and Terrible Majesty of the Towering Seas of Ire That Are Themselves the Milquetoast Shallows Fringing My Vast Oceans of Wrath (It may be somewhat uncertain of its abilities, but it proceeds to remind an unimpressed combatant that Culture Ships/Minds are not generally given to _boasting_.)
- Disaster Scavengers: A vocation that entire civilizations engage in. Species such as the Liesieden and the Ronte get a technological leg up by picking over the bones left behind by Subliming civilizations, and conflicts between scavengers are a considerable source of galactic conflict.
- Human Aliens: The Gzilt, like many other species in the Culture universe.
- Mildly Military: This is the "hat" of the Gzilt. Citizenship is tied to military service à la Starship Troopers, but every member of society serves in the military forces at some stage in some way, and holds a lifelong reserve position afterward, and it has been a long time since the Gzilt have engaged in any kind of combat.
- Multi-Armed Multitasking: The instrument used to play the Hydrogen Sonata requires either two very well-coordinated musicians or one with four arms. Vyr Cossont went with the latter option, much to her parents' dismay as body mods were unfashionable in Gzilt society at the time.
- Names to Run Away from Really Fast: The Mistake Not...
- Overly Long Name: The Mistake Not..., whose full name is an in-joke among the Minds, and is abbreviated to preserve its in-joke status.
- Also, T. C. Vilabier's 26th String-Specific Sonata For An Instrument Yet To Be Invented, catalogue number MW 1211 (commonly referred to as "The Hydrogen Sonata")
- "Shaggy Dog" Story: In-Universe; the Culture have already guessed what the big secret is about, and that they're not likely to do anything if they find out the truth, so why bother going to all the trouble? Yet they do anyway.