| The Nightmare Stacks | |
 | |
| Type | novel |
| Timeline Order | |
| Preceded by The Annihilation Score |
Followed by The Delirium Brief |
On a parallel-universe Earth, a species called Elves or alfär have evolved to be expert magic users. They have visited Earth in the past, from which comes a great deal of folklore. Civil war has left the Elves' home world uninhabitable and they plan to magically invade the Earth and make it their new home. To scout ahead, they send Agent First of Spies and Liars, the first daughter of the Elven King, who takes over the human identity of a student named Cassie. Agent First enters a relationship with Laundry member and PHANG Alex Schwartz, intending to sacrifice him to infiltrate and defeat the Laundry from within. Unbeknownst to her, Alex was himself an unwitting honeytrap set by the Laundry, and their meeting leads to Cassie throwing in her lot with the human race.
The Elves invade Leeds and threaten the British heartland with their powerful magic, though they are defenseless against non-magical human weapons. Alex and Cassie defeat them with the aid of returning characters Pete and Pinky, among others. Cassie immediately surrenders to Alex, declares the Elves to be refugees who cannot go home for fear of their lives, and requests asylum under the Immigration, Asylum and Nationality Act 2006.