Slow Horses glossary

Leadership

What is First Desk—and is it real?

In Slow Horses, First Desk is the top leadership position inside the Security Service. The person holding it controls senior appointments, major operational decisions and the official version of events. Second Desk is the powerful deputy role immediately below it.

Fact versus fiction: “First Desk” is the terminology of Mick Herron’s fictional Service world, not the public title used for the real head of MI5. MI5 publicly calls its head the Director General. The programme uses its own labels to make hierarchy and rivalry immediately legible.

MI5: who we are and Director General ↗

Security testing

What is a tiger team?

A tiger team is an authorised group assembled to test whether an organisation, building or system can be breached. The term exists outside the series and is used in security, engineering and problem-solving contexts.

In Series 3, the phrase describes a private team hired to penetrate the Park. The dramatic question is not merely whether the test works, but who designed it, what the stated objective conceals and when an exercise becomes a genuine threat.

Series 3: Real Tigers →

Secure communication

What is a flash box?

Within the programme, a flash box is a secure emergency communication device or channel used for urgent Service business. Searches also commonly spell it as one word: “flashbox”.

The screen term signals that a message carries exceptional priority. It should not be treated as a public guide to current MI5 equipment: operational technology and procedure are deliberately not described by the real organisation in television-level detail.

Identity

What is a cold body?

A cold body is a false identity built deeply enough to withstand scrutiny. It is more than a forged passport: the supporting records and history are intended to make the person appear real when checked.

Series 4 turns that idea into a central question about who created particular identities, why they were authorised and what old Service decisions have survived into the present.

Series 4 Episode 1: “Identity Theft” →

Method

What does tradecraft mean?

Tradecraft is the practical collection of methods used in intelligence work: surveillance, countersurveillance, cover, secure communication, recruitment and the handling of information or sources.

The series uses good and bad tradecraft as character evidence. Technique matters, but judgement matters more; apparently polished officers can misread a situation while the supposedly failed agents notice the human detail.

Series 1 Episode 3: “Bad Tradecraft” →

Internal security

Who are the Dogs?

The Dogs are the Service’s internal security and enforcement officers in the fictional world. They guard sensitive facilities, pursue suspected internal threats and carry out orders from senior management.

The nickname also communicates how they are used: disciplined, intimidating and expected to chase whoever the Park points at—whether or not the Park has told them the whole story.

Institution

What is “the Park”?

Regent’s Park, usually shortened to “the Park”, is the series’ name for MI5 headquarters and the senior organisation around it. It contrasts polished authority with the neglect and clutter of Slough House.

The real MI5 headquarters is Thames House in Westminster. The programme’s Park is a fictional screen geography assembled through sets and multiple locations.

Explore the screen geography → · MI5 information ↗

People

What is a joe?

“Joe” can refer to an agent operating in the field or, depending on context, a human source handled by an officer. The intended meaning comes from who is speaking and what relationship is being described.

The ambiguity suits a world where people are routinely discussed as operational assets rather than complete human beings.

Cold War file

What is a cicada?

In Series 2, “cicada” refers to the idea of a sleeper agent: someone embedded in ordinary life, apparently dormant, until activated for an operation.

The image is useful because cicadas remain unseen for long periods before emerging. The series uses the concept to test whether an old intelligence story is evidence, obsession or deliberate misdirection.

Series 2: Dead Lions →

Institutional survival

What are London Rules?

In Herron’s world, London Rules express the instinct to protect yourself when an operation fails and blame begins moving through the institution. The phrase is less a formal handbook than a cynical survival principle.

It becomes both a book title and the source material for Series 5, where political management and operational response repeatedly pull in different directions.

Series 5: London Rules →

Exchange

What is a dead drop?

A dead drop is a method of leaving material at an agreed place so two parties do not need to meet directly. Timing, concealment and knowing whether a location is being watched are part of the risk.

The principle is real and historically well documented, but any screen version is shaped for clarity and pace.

Compromise

What is a burn notice?

In spy-fiction language, a burn notice warns that an identity, source or officer has been compromised and should no longer be trusted or used in the same way.

Exact organisational terminology varies. On this site the phrase is explained as programme language, not presented as a current MI5 form or publicly confirmed procedure.