Lamb liaises with a Russian defector who may hold a clue to sleeper agents. River is on the trail of a ruthless assassin.

Louisa makes her move on Pashkin. Catherine makes moves of a different kind when she plays high-stakes chess with a sinister stranger.

Lamb orders his Slow Horses to walk into traps. River believes something alarming is heading toward London.

Lamb and Katinsky go head-to-head. Ho finds himself trapped on a train with a Russian assassin.