Slough House is an administrative purgatory for MI5 service rejects who have bungled their job but have not been sacked. Those consigned there are known as “slow horses”. They are expected to endure dull, paper-pushing tasks, along with occasional mental abuse from their miserable boss, Jackson Lamb, who expects them to quit out of boredom or frustration. Life in Slough House is defined by drudgery. Yet the Slow Horses somehow get involved investigating schemes endangering Britain.
- Gary Oldman as Jackson Lamb, the head of Slough House and slovenly, rude, and apparently drunk most of the time. Yet he has a sharp and devious mind, and retains his abilities as an experienced intelligence officer.
- Jack Lowden as River Cartwright, an up-and-coming MI5 agent shunted aside to Slough House after a very public training-exercise mistake.
- Kristin Scott Thomas as Diana Taverner, the deputy director-general of MI5 and head of operations and designated “Second Desk”.
- Saskia Reeves as Catherine Standish, the office administrator and a recovering alcoholic.
- Olivia Cooke as Sidonie “Sid” Baker (season 1), a competent MI5 agent inexplicably assigned to Slough House.
- Rosalind Eleazar as Louisa Guy, assigned after a tail operation went badly.
- Christopher Chung as Roddy Ho, an obnoxious computer expert and former hacktivist.
- Steven Waddington as Jed Moody (series 1), an ex-member of “The Dogs”, an MI5 internal affairs and tactical unit.
- Dustin Demri-Burns as Min Harper (series 1–2), assigned after leaving a top-secret disk on the train.
- Paul Higgins as Struan Loy (series 1), assigned after sending an inappropriate work email.
- Aimee-Ffion Edwards as Shirley Dander (series 2-present),[5] who has anger issues.
- Kadiff Kirwan as Marcus Longridge (series 2-present),[5] who has gambling issues.