Susie's return to the Slough

Lore & Background

Susie Riedenschneider enters the Slow Horses world as a contrast to everything the Slough represents. Where Jackson Lamb drinks at his desk, ignores protocol, and treats intelligence work as a kind of street fight, Susie arrives with the polished, procedural confidence of an officer who still believes the institution will reward competence and punish malfeasance. Her initial interactions with the Slough are marked by friction: she sees them as a embarrassment, a bureaucratic afterthought; they see her as a liability who hasn't yet learned that the paperwork doesn't save you when the real threat is in the room.

Her 'return' to the Slough is less a physical reassignment than a narrative one. As her own case unravels and the channels she trusted prove compromised or indifferent, the Slough becomes the only unit willing to do the unglamorous, dangerous, off-the-books work that actually solves the problem. This mirrors a recurring theme across Mick Herron's novels: the Slough is where MI5 sends the problems it doesn't want to look at, and also where the problems that MI5 can't solve eventually land. Susie's arc crystallises that duality.

What makes her story resonate with fans is the emotional honesty of her transition. She doesn't become a Slough loyalist overnight; she keeps her instincts, her standards, her suspicion of Lamb's methods. But she learns to work within the chaos rather than against it, and in doing so she gives the Slough something they rarely have: a reason to explain themselves, to justify their existence to someone who still cares what MI5 is supposed to be.

In Their Own Story

The fluorescent lights in the Slough corridor hum at a frequency that makes your teeth ache. Susie stands outside the glass door, watching Jackson Lamb argue with a phone in one hand and a half-eaten sandwich in the other. The office beyond is a graveyard of cold coffee cups and post-it notes that say things like 'CALL BACK. DO NOT CALL BACK. CALL BACK BUT ANGRY.'

She knocks. He doesn't look up.

'You're in my way, Riedenschneider.'

'I'm in your corridor, Lamb. Which, technically, is MI5 property.'

He hangs up. Looks at her. The sandwich is now a smear of mustard on his sleeve. 'And what does MI5 property want with Slough property?'

She sets a manila folder on his desk. Doesn't smile. 'The thing you don't want to know about. The thing that's going to make your Tuesday very, very long.'

He picks up the folder. Reads the first line. Sets it down. Picks it up again. 'You're either very brave or very stupid.'

'In my experience, sir, those are the same thing in this building.'

He laughs. It's not a kind laugh. But he pulls out a chair.

Reader's Guide

Season 4 opens with Susie operating within MI5's normal chain of command, handling a case that appears straightforward. Early episodes establish her as competent but constrained by institutional politics; her superiors are more interested in optics than outcomes.

As the investigation deepens, cracks appear. Information she's been given is incomplete or deliberately misleading. The people she reports to begin distancing themselves. It's at this pressure point that the Slough becomes relevant—either through a direct referral, a shared suspect, or a piece of intelligence that only Jackson's team has been tracking.

The middle stretch of the season is where the friction peaks. Susie and Lamb clash over methods: she wants evidence, procedure, a clean file; he wants results, even if the file looks like it was assembled by a raccoon. The Slough team—Wendy, Liza, Dougie, and the others—react with a mixture of amusement and wariness to having a 'proper' officer in their orbit.

The resolution sees Susie making a choice that costs her institutional standing but saves the day in a way the system never would have. She doesn't join the Slough permanently, but the door is left open. Jackson's final line to her carries the weight of a man who's seen a hundred officers come through that corridor and very few leave changed.

Chronologically, Season 4's events build on the institutional tensions seeded in Seasons 2 and 3, where the Slough's existence was repeatedly threatened by MI5's leadership. Susie's arc is, in a sense, the personal cost of that institutional instability made visible to a character who was still inside the machine when it started to grind.

Did You Know?

• The Slough Department's name is a deliberate inversion of the town of Slough in Berkshire, a place historically associated with being overlooked and undesirable—fitting for MI5's dumping ground for misfit officers.
• Jackson Lamb's office in MI5 headquarters is famously cluttered and chaotic, contrasting sharply with the sterile, controlled environments of the rest of the building—a visual shorthand for the Slough's philosophy.
• Mick Herron's original novels, on which the series is based, feature the Slough (called the Slough in the books) as a recurring institution across multiple books, giving the TV adaptation a deep well of backstory to draw
• The show's title 'Slow Horses' is a real MI5 slang term for officers who have been 'dumped'—removed from active intelligence work for various infractions—and assigned to a low-prestige department.