by Philip Chambers
Designed by Philip Harrison
Directed by Raymond Menmuir
A young woman (Edina Ronay) in diving gear breaks into the top-secret defence installation, "The Nutshell", and photographs the uber-secret file "Big Ben".
Steed and Mrs Gale are called in to investigate and, after passing the security desk manned by Bill (Edwin Brown) are led by Laura (Patricia Haines) to meet the shadowy DISCO (John Cater).
Cathy is assigned to work with Security Officer Venner (Charles Tingwell) and his assistant Susan (Christine Shaw) to search the archives for any trace of the intruder, whose face was clearly photographed, while Steed is sent to Special Branch.
Instead, Steed goes to the woman's apartment and, being on good terms with her, asks for the microfilm in return for a large bundle of pound notes. Back at his apartment, Cathy is stunned to hear him call Disco and tell him he's just leaving Special Branch and has identified the woman as Elin Strindberg, a Swedish cabaret performer. She's more amazed when he pulls out a secret short-wave radio and contacts a double-agent called Jason (Jan Conrad), and arranges to meet him at a nearby airfield.
Back at The Nutshell, Susan rings a man called Alex (Ray Browne) and tells him to 'see her friend' at the same address as Steed had supplied.
Disco is suspicious of Steed, and orders Venner to talk to him - Cathy tells Venner of the rendezvous and he and Anderson (Ian Clark) race to intercept it. Steed holds them at gunpoint until Jason escapes, then turns himself in.
He's taken back to The Nutshell and interrogated and electrocuted, but reveals nothing. There's a temporary outage, and Venner tips his hands, offering Steed £30,000 for the microfilm. Steed demands £50,000 and they agree on an escape plan. Meanwhile, Cathy interrupts Alex searching Steed's flat, and she rounds up him and Susan. She forces Alex to tell Disco the ringleader's name - Venner. Disco then orders the guards re-apprehending Steed to capture Venner instead.
The criminals rounded up, Steed reveals that Jason told him of someone hiring Elin to break in, paid her a bit more to double-cross them. Furthermore, there was never a threat to national security, Elin had used a camera with pre-exposed film - shots of Steed and Cathy on holiday!