"The Nutshell" is the fourth episode of the third series of the 1960s cult British spy-fi television series The Avengers, starring Patrick Macnee and Honor Blackman. It was first broadcast by ABC on 19 October 1963. The episode was directed by Raymond Menmuir.

Steed and Cathy investigate espionage by a pretty burglar in "The Nutshell", codename for a top-secret underground nuclear shelter. They are not telling each other everything, and somewhere a traitor lurks.