The Doctor and Sam travel to 2108 and find that Earth is now Paratractis, a Tractite colony. The Doctor realises history has been changed, and Jo Grant gets caught up in this as well.
Tropes present in Genocide include:
- Apocalypse How: The Tractites creating an alternate reality threatens to pull the Multiverse into the Time Vortex.
- Canon Discontinuity: The depiction of Jo in this book is difficult to reconcile with her later appearance in "Death of the Doctor". We can perhaps imagine that this book takes place in a "rough patch" of Jo's marriage, but she is well and truly on her own here. The word "divorce" doesn't appear in the book, but it's clear she has total financial responsibility for her child and herself. Moreover, she's only got one child in this story, Matthew, who just started high school. Death to the Doctor says she's got seven kids. It's hard to see how she could have six more kids after Matthew, with Cliff, starting at roughly the age of thirty-five. Additionally the separation/divorce from Cliff has left her having to hold down two jobs in Hackney. How exactly she was supposed to be globetrotting given this bleak reality is unclear.
- Cold-Blooded Torture: What the Tractites do to the Doctor.
- The Dreaded: The Uncreator, who will one day return to wipe the Tractites of Paratractis out.
- Earth All Along: Paratractis. Slightly averted as the Doctor works out it's Earth quickly.
- The Empire: The Earth Empire.
- Genocide Dilemma: A major part of this story. If the Doctor restores history he will effectively be doing this to the Tractites. Slightly subverted in the end: though he can't prevent the invasion he is able to give the survivors their freedom.
- The High Queen: The Earth Empress who agrees to grant Tractis independence.
- Humans Are Bastards: The humans attacked Tractis, killing or enslaving many of the natives.
- Kill All Humans: What the Tractites want to do.
- Make Wrong What Once Went Right: The Tractite plan to wipe out humanity. However it poses a big dilemma in morality for Sam as she sees how good the world is under them and hears how the humans attacked them.
- Nice Job Breaking It, Hero: Sam letting Kitig into the TARDIS almost gets the Doctor killed.
- Our Minotaurs Are Different: The Tractites look part-ox.
- The Plague: How a Tractite left in the present and Jacob try to wipe out humanity.
- The Slow Path: The TARDIS has to take this through 1.07 million years when it is thrown into a volcano.
- Timey-Wimey Ball: Very much so. A timeline that shouldn't exist, being foiled by a Stable Time Loop.
- Write Back to the Future: On the Doctor's instructions Kitig leaves these to tell Sam and Jo to find the TARDIS. After the Tractites are stopped he spends nearly all the rest of his life making the messages to make sure they are found.
