When a young man is kidnapped and threatened with beheading, the slow horses find themselves unexpectedly embroiled in a dangerous case that forces them to prove their worth.

Long-buried Cold War secrets emerge which threaten to bring carnage to the streets of London. When a liaison with Russian villains takes a fatal turn, our hapless heroes must overcome their individual failings and raise their spy game in a race to prevent a catastrophic incident.

A romantic liaison in Istanbul threatens to expose a buried MI5 secret in London. When Jackson Lamb and his team of misfits are dragged into the fight, they find themselves caught in a conspiracy that threatens the future not just of Slough House but of MI5 itself.

In season 4, a bombing detonates personal secrets, rocking Slough House's already unstable foundations.