When Ellery calls Inspector Queen to report that the victim has disappeared while dressed as the Mad Hatter, Inspector Queen scoffs at the idea. David Wayne played The Mad Hatter in the 1960's BATMAN TV Series.
The only one of the episodes (excepting the pilot) directly based on material written by Dannay and Lee.
A clever joke was inserted for those who know their movie history. Howard Biggers mentions a phony 7-year contract Emmy Lockridge was forced to sign "and that picture-part you had to turn down last month." When asked if that was true, Emmy replies "It was a dreadful part, with no lines. Some deaf and dumb farm girl who gets forcibly compromised. Well, they gave it to Jane Wyman. I didn't want to play it anyway." The role in question was for the film Johnny Belinda (1948) and won Jane Wyman the Oscar for Best Actress in a Leading Role.
As in other episodes of the series, several of the cast are named for notable mystery authors. Here they are: Lockridge (for Richard and Frances Lockridge), Biggers (Earl Derr Biggers), Gardner (Erle Stanley Gardner), Doyle (Arthur Conan Doyle), Allingham (Margery Allingham), Reinhart (Mary Roberts Reinhart) and Carr (John Dickson Carr).
This title is based on an event found in Lewis Caroll's book 'Alice In Wonderland'.