Spyfall, Part 1

The security of the entire world is at stake, so the head of MI6, C, enlists the Thirteenth Doctor and her team to investigate former agent spy Daniel Barton to see if he's been turned against them. Meanwhile, an alien threat that can pass through walls, even those of the TARDIS, known as the Kasaavin is wiping out spy organisations all over the planet. Can former MI6 agent "O" help them, or is he not who he claims to be?