An attractive young woman penetrates the security of The Nutshell, a secret underground government facility, and photographs Big Ben... a secret file listing the names of all double agents. Steed and Cathy are called in, but as their investigation progresses all evidence points to Steed... Has he gone over to the "other side"???

In my opinion, this may be the best of the Cathy Gale episodes if not one of the best episodes of the entire series. It has a great plot, great guest actors, and really elaborate sets for the budget.

In the opening scene, the camera that eventually catches Elin can be seen being lowered into the set. Later when Disco is showing Big Ben to Steed and Cathy, the camera is no longer there.

Also while Disco is explaining Big Ben, the shadow of the boom mike can be seen moving across Steed's face.

Between the time that Steed tells Elin "Good luck" and Elin returns the favor, the camera bumps into something, jars, and a thump is heard.

When Venner blanks out the cameras in The Nutshell, the static can be heard several seconds before it appears on the first monitor.

After Anderson is notified where to find the microfilm in Steed's flat, he leaves the living room. As Cathy confronts him, a shadow of someone in the now empty living room can be seen by the front door.