The Unnoticed are bound to keep themselves isolated from all history, or face a complete collapse from existence.
The Book of the Still is a lifeline for stranded time travellers -- write your location, sign your name and be instantly rescued. When the Unnoticed learn that within the book someone has revealed both their existence and whereabouts they are forced into murderous intercession to find it.
Fitz knows where it is, but then he’s the one who stole it. Carmodi, addicted to the energies trapped in frequent time travellers, also knows where it is. But she’s the one who’s stolen Fitz. Anji, alone on a doomed planet, trying to find evidence of a race that has never had the decency to exist, doesn’t know where anybody is.
Embroiled in the deadly chase, the Doctor is starting to worry about how many people he can keep alive along the way...