Louisa is an intelligence agent exiled to Slough House after her career derailed. She shares a small office with Min Harper and harbors a strong desire to return to mainstream intelligence work at Regent's Park. When assigned to vet a Russian delegation by James Webb, she views the task as her primary ticket out of career exile.

Jackson is the head of Slough House, the dumping ground for MI5 operatives who make career-ending mistakes. A veteran of the Cold War espionage scene in Berlin, he projects a cynical, abrasive, and unhygienic public persona that masks sharp intelligence. He initiates an off-the-books investigation when a former colleague dies under suspicious circumstances.

River is a young MI5 agent banished to Slough House after failing a public training exercise. Raised on his grandfather's stories of Cold War glory, he yearns for real action and eagerly accepts an undercover assignment in the quiet town of Upshott. He struggles with the gap between his romanticized view of espionage and the mundane reality of his career.

Katinsky is an elderly former Soviet cipher clerk who lives in reduced circumstances in London, operating a fake school called the Wentworth Academy. He suffers from liver cancer and possesses knowledge of old Soviet intelligence operations, specifically relating to sleeper agents known as "cicadas."

Min is an agent exiled to Slough House after a humiliating public error involving classified documents. He lives in a small apartment following his divorce and struggles with alcohol use. His new assignment tracking a Russian delegation offers him a chance at professional redemption.

Catherine is an administrative assistant at Slough House with a history of alcohol use disorder. She uses her keen understanding of human behavior to manage the eccentric personalities of the slow horses, often acting as a stabilizing force and remote handler for field operations.

Ho is the resident tech expert at Slough House. He completely lacks social skills and actively dislikes most of his colleagues, preferring to spend his time writing scripts to fake his own productivity and interacting with women online under false pretenses.

Shirley is a new addition to the slow horses, exiled for punching a superior who sexually harassed her. She struggles with a cocaine habit and resents the intelligence bureaucracy, yet she retains strong investigative instincts and a volatile temper.

Marcus is a newly exiled agent in his mid-forties whose career was derailed by a severe gambling disorder. He brings practical operational experience to the team and acts as a stabilizing physical presence during field assignments in London.

Webb, nicknamed "Spider," is an ambitious MI5 agent at Regent's Park who lacks interpersonal skills and is widely distrusted. He uses the Slough House agents to bypass standard budgetary oversight for a high-profile recruitment operation involving a Russian oligarch.

Pashkin is a wealthy Russian oligarch visiting London with an interest in establishing covert ties with British intelligence. He travels with a dedicated security detail and insists on strict operational secrecy regarding his meetings in the Needle skyscraper.

Taverner, known as the Second Desk of MI5, is a high-ranking intelligence official. She is highly ambitious, deeply involved in office politics, and forced to tolerate Jackson Lamb because he holds compromising information about her past operations.

Dickie is a washed-up, elderly former MI5 agent who worked in Berlin during the Cold War. His sudden death on a replacement bus service initially looks natural, but it sparks Jackson Lamb's off-the-books investigation into old Soviet operations.

Kelly is a young woman living in the quiet town of Upshott. She works at the local pub, sketches aerial maps, and spends her afternoons piloting a small plane at the local flying club. She develops a romantic connection with River Cartwright during his undercover mission.

David Cartwright, affectionately known as the "O.B.," is a retired, highly respected intelligence operative. He provides his grandson River with historical context regarding Soviet spymasters and Cold War tactics.

Andrei is a bald Russian operative. Dickie Bow recognizes him in London, prompting a pursuit that ends in Dickie's death.

Piotr is a member of Arkady Pashkin's Russian delegation. He handles the bulk of the communication during security meetings and maintains a watchful eye over the operation's logistics.

Kyril is part of Pashkin's security team. He frequents London establishments like hookah lounges and easily spots when he is being followed by amateur operatives.

Griff is a local resident of Upshott who knows the secret entrances to the abandoned military facilities. He resents River's presence in the town.

Molly is the gatekeeper of MI5's oldest secrets in the records department. She uses a wheelchair and is one of the few people at central headquarters who maintains a friendly rapport with Jackson Lamb.

Nick is the head of the "Dogs," MI5's internal security and disciplinary unit. He shows little interest in investigating incidents involving Slough House personnel until pressured.

Sam is a former Head Dog at MI5 who now works as a private investigator following a scandal involving missing funds. He quietly supplies Jackson Lamb with off-the-books research.

Cal is a private security guard working at a digital archive facility. His shift is interrupted by a mysterious power outage and an intruder.