A novel by Diana Wynne Jones. A team of mages finds it necessary to invade an alternate universe.
- The Alleged Car: Tony's Deux Chevaux, which Tod mentally labels a "subcar".It was small. It had a domed top, like the head of an amiable but stupid dog, and a curious posture, down at the front and up at the back, as if the stupid dog were engaged in sniffing the guger; and to make it more remarkable, it was not one colour but several random ones. The domed top was orage. One flimsy-looking door was green. The down-bending bonnet was sky-blue. The rest was a rusty sort of cream. It was like a jester—or someone's idea of a joke.
- Baby Talk: Marcus, Zillah's son. Rather well educated and smart even though his words hardly make sense to anyone but Zillah.
- Elseworld: Earth, Arth, and Petarchy. They even have alternate people called Analogues.
- Everything's Better With Ether Monkeys: Jimbo
- Henpecked Husband: Mark tends to question from time to time why he married Paulie.
- I Choose to Stay: After the congo dancing scene, the women decided to stay in Arth
- The Infiltration: What drives the plot; realizing that another world has been forcing Earth into war after war to force them to build new technology and copy it, Gladys sends a team of highly trained female mages (as well as Zillah, by accident) to the other world and stop them.
- The Mole: Brother Tony is assigned to spy on Paulie and return information to Arth. Later, because Tod kissed Zillah, he has to fill in, much to his displeasure.
- Never the Selves Shall Meet: When they enter the alternate universe, all the ones with counterparts there instantly die.
- Sex Is Evil: In-universe. It's not allowed in Arth because it will upset the balance.