According to "The Prisoner: The Official Companion to the Classic TV Series" by Robert Fairclough, the strain of filming this episode caused Leo McKern to suffer either a nervous breakdown or a heart attack (accounts differ), forcing production to stop for a time.
A number of fans who speculate that Number Six is in fact John Drake from Patrick McGoohan's earlier series, Danger Man (1960) usually reference this episode. They claim that Number Two says "Report to my study in the morning, Drake!" when in fact he really says "Report to my study in the morning break."
This was originally scheduled as the final episode of the first season. However, after the show was canceled, Patrick McGoohan used it as the springboard for the final episode he shot to sum up the story.
The script was originally credited to "Archibald Schwartz". Patrick McGoohan used a pseudonym because he felt that the script would be ridiculed.