1. Something from the Nightside
2. Agents of Light and Darkness
3. Nightingale's Lament
4. Hex and the City
5. Paths not Taken
6. Hell to Pay
7. The Unnatural Inquirer
8. Just Another Judgement Day
9. The Good, the Bad, and the Uncanny

The series takes place in a fictional inner city area of London called "The Nightside". Elements of fantasy, science fiction and the supernatural all feature heavily in both the Nightside itself and its inhabitants.

Indicated by its name, the Nightside experiences perpetual night ("it's always 3am"). The Nightside itself is contained within London, yet is larger than London by a significant measure. Though access to the Nightside is hidden, it does experience a steady stream of tourists from the "normal" world.

The main character, John Taylor, describes the Nightside "as a place where dreams come true and nightmares come alive. Where one can buy anything, often at the price of your soul... or someone else's. Where the music never stops and the fun never ends". Numerous seemingly impossible services or goods are available-often quite commonly-in the Nightside; examples include stores selling albums from alternative time-lines (e.g. a live Mama Cass concert from 2003), a business where customers pay to be possessed for a brief period of time "just for the kick of it," and a drug called Revert which causes users to temporarily devolve into a biological ancestor (e.g. a Neanderthal).

A recurring feature in the Nightside is the appearance of "Timeslips", locations where the Nightside has collided with another part of space and/or time, and sometimes with a completely separate, alternate timeline as well. A number of characters identified within the series are "refugees" displaced by Timeslips. The appearance of Timeslips is largely unpredictable, though in some cases Timeslips have been predicted or even deliberately created.

The Nightside is overseen by The Authorities, a committee whose members are largely unknown which makes rules, policy, and decisions concerning what is allowed to happen within the realm. The Authorities' decisions are enforced by their Agent (or "The Man"), who among other equipment and abilities is given The Voice, a way of issuing vocal commands which no human can refuse. At the beginning of the first book this position is held by Walker.