Slow Horses is a British espionage series based on Mick Herron’s novels. The show follows a team of MI5 misfit agents at Slough House as they navigate political pressure, inter-agency politics, and dangerous operations. Key points about the cast

• Jackson Lamb, played by Gary Oldman, is the abrasive head of Slough House who leads the group of agents otherwise sidelined from the main service.
• River Cartwright, portrayed by Jack Lowden, is a young MI5 operative sent to Slough House after a training error spirals out of control.
• Diana Taverner, the Deputy Director General and head of operations, is a central figure in the MI5 hierarchy (portrayed by Kristin Scott Thomas).
• Ingrid Tearney, later revealed as First Desk, is the Director General of MI5 in certain seasons (Sophie Okonedo).
• David Cartwright, River’s grandfather and a retired senior MI5 officer, appears as a high-level figure with a complex past (Jonathan Pryce).
• The ensemble also includes supporting regulars like Catherine Standish (Saskia Reeves), Louisa Guy (Rosalind Eleazar), Roddy Ho (Christopher Chung), and Shirley Dander (Aimee-Ffion Edwards), among others who join or recur across seasons.
• New season additions bring fresh characters such as J.K. Coe (Tom Brooke) and Zafar Jaffrey (Nick Mohammed), expanding the MI5 landscape and Slough House dynamics.

Seasonal dynamics

• The cast evolves across seasons, with some characters becoming central figures while others depart or shift roles. Key players typically include Lamb, Cartwright, Taverner, and Standish, with recurring appearances by Lieutenants or operatives connected to Slough House.
• The show also introduces new antagonists and allies as plots escalate, reflecting evolving tensions within the intelligence community.

If you’d like, I can tailor a quick guide to each season’s principal cast and character arcs, or provide a handy cast grid highlighting who’s in Slough House versus the wider MI5, including which actors joined in later seasons.