(Kristin Scott Thomas): The MI5 Deputy Director-General who must handle the "Code September" threat while managing political fallout.

The episode picks up with in a dire situation at an airfield in Upshott. He discovers that Alex Tropper , seemingly a quiet housewife, is actually a Russian sleeper agent. She subdues River and prepares to fly a plane laden with explosives toward the Glasshouse in London, where a major anti-capitalist protest is taking place.

(Jack Lowden): After being outmaneuvered in Upshott, he desperately tries to warn MI5 about the incoming plane.

In London, infiltrates the MI5 archives at Regent's Park to uncover the identity of the Russian spymaster. He realizes that Nikolai Katinsky , whom he previously dismissed as a minor player, is actually the mastermind who has been targeting Slough House for personal revenge.

Meanwhile, and Marcus Longridge attend a high-stakes meeting at the Glasshouse with Arkady Pashkin and James "Spider" Webb . The meeting quickly turns violent when Pashkin shoots Webb, leaving the agents trapped as the building begins to evacuate due to a "Code September" alert. Key Cast and Characters

(Rade Šerbedžija): Revealed as the spymaster orchestrating the "Cicada" sleeper agent plot.