After the events of Speaker for the Dead, the human colony on Lusitania has cut ties with Starways Congress and is living openly with the Pequeninos and newly resurrected Formics. Not liking the rebellion and fearing that the deadly Descolada virus native to the planet might get off world, the Congress launches a fleet with the Little Doctor device on it. Ender and his friends have to stop the fleet from bombing Lusitania, eventually, in the next book. Plus other subplots.
Xenocide provides examples of:
- Bread, Eggs, Milk, Squick: One of the mentioned methods that the godspoken use to commune with the gods is "checking for accidental murders". This is in a list with "doorway-standing" and "counting multiples of five".
- Brother–Sister Incest: Probably nonexistent, but rather maliciously suggested by New Peter, after Ender's soul accidentally creates teenage versions of his siblings from Outside the universe, with New Valentine being inaccurately saintly and beautiful.
- Chekhov's Gun: Wang-mu's dream of being the wife and companion of the long-dead Peter the Hegemon.
- Decoy Protagonist: The real protagonist of the Path story thread isn't Qing-Jao, it's Wang-Mu.
- Deus ex Machina: Outside solves all the major problems of the story, though it causes a few of its own.
- Qing-Jao's searches for information discover Demosthenes' identity, i.e. one of the greatest mysteries of the past few millenia, with pretty much no explanation behind it.
- FTL Test Blunder: Faster-than-light travel becomes possible through the means of an artificial intelligence who resides outside the known universe (Jane) taking control of a transport ship and moving it from one point in the universe to Outside and then back to its destination. The travel is instantaneous, but on the first trip residual matter in the Outside is accidentally formed into what the travelers are imagining, resulting in the creation of new people. Miro, who is paralyzed, imagines his pre-crippled body and exchanges into it while his old body dies, while Ender, who has long felt overshadowed by his brother and sister, creates clones of them that share his soul.
- Genetic Engineering Is the New Nuke: The "godspoken" of Path are hyperintelligent OCD sufferers.
- Genius Serum: It becomes a major concern of the piqueino species that their intelligence is simply the byproduct of the descoloda virus infecting their world. This especially when the human scientists figure out a way to make a non-lethal version of the virus that would allow the piqueinos to safely leave their potentially doomed planet. Planter demands his friends remove the virus from his body, suffering in agony as its loss begins to kill him, but crucially remaining lucid and intelligent all the way to his end, proving the pequeinos are a sapient species in their own right.
- Genocide Dilemma: More important now than it was in Ender's Game. It's even in the title, which is a portmanteau of "xeno" (aliens) and "genocide".
- Go Mad from the Revelation: Qing Jao goes mad about a quarter of the way through Xenocide and never goes back. Ends up being an Ironic Hell, as she's viewed slowly as the most holy person in all the Hundred Worlds eventually.
- Humans Are Bastards: There's the Xenocide fleet (though that at least has the justification of trying to destroy an unstoppable plague), the fact that the god-touched are actually genetically engineered for OCD that activates if they have treasonous thoughts, the way that the Peter and Valentine clones are treated...
- I Die Free: After discovering that the descolada virus is not native to Lusitania and may be responsible for pequenino evolution, Planter insists on being placed in the quarantine room where humans are cleansed of the descolada, so he can prove the virus is not what makes piggies sentient. Since his body still needs the descolada to live, though, he suffers the entire time, but dies glad that his mind is truly his own and not that of a parasite.
- Idiot Ball: Qing Jao has one superglued to herself, justified because of her devotion to the gods.
- Missing Mom: Han Qing-jao.
- Named After Somebody Famous: Qing-jao's mother Jiang-qing; Jiang Qing and the "Gang of Four" are remembered fondly in Path's version of history. Qing-jao herself is named after a poet.
- One-Word Title
- Twisted Eucharist:Ela and her assistants simply brought several hundred sugar cubes impregnated with the viricide bacterium, and as many vials of the solution containing the recolada. They were passed among the congregation, and each of the pequeninos took the sugar cube, dissolved and swallowed it, and then drank off the contents of the vial.
"This is my body which is given for you," intoned Peter. "This do in remembrance of me."
"Have you no respect for anything?" asked Ender.
"This is my blood, which I shed for you. Drink in remembrance of me." Peter smiled. "This is a communion even I can take, unbaptized as I am."
"I can promise you this," said Ender. "They haven't invented the baptism yet that can purify you."
- Two-Part Trilogy: Ender's Game began as a stand-alone short story, then was later expanded into a novel. The novel is also sufficiently stand-alone, but the final chapter does have a sequel hook that allows for a sequel if you choose to read it. The sequel also sits surprisingly well as a stand-alone conclusion to Ender's story, but also has a sequel hook if you want to tie up some below-the-surface loose ends. This is where it gets into Two-Part Trilogy country. Xenocide and Children of the Mind, are far more connected than the previous books and were originally intended to be a single volume, but were broken off into two with a superficial cliffhanger between them. Children of the Mind returns to being a suitable conclusion, if you count the main character Ender dying, but only opens up the biggest cliffhanger in literature since Chapterhouse: Dune. Like the Dune series, it's near impossible to differentiate between the overlapping Sequelitis, Two-Part Trilogy, and Trilogy Creep.
- What Happened to the Mouse?: Jakt and his trouble in adapting to life on Lusitania get a few mentions early on and then are promptly forgotten.
- What Measure Is a Non-Human?: Zig-zagged with the philotic creations. Jane is treated in full accordance with the Hierarchy of Foreignness as raman, a friendly intelligent non-human that can be reasoned with, as are the buggers even after the revelation that Hive Queens are possessed by philotes from Outside rather than actually born intelligent. Yet, the remade Peter and Valentine summoned from Outside by Ender during the FTL test jump are immediately treated as subhuman, inhuman abominations, by virtually all characters, including themselves.