A McGuffin, in movie parlance, is a plot-driving device. It can be anything from a mis-shaped crystal skull to (in this case) a simple finger ring.
On a dark and stormy night at the Carlton, Paladin finds his latest prospective conquest hijacked by a flamboyant member of Russia's ruling Romanov clan, who glibly quotes Shakespeare's "Two Gentlemen of Verona" - a quote Paladin ruefully equates instead with "Love's Labours Lost".
He is interrupted during a lavish (but lonely dinner) by a South Sea Beauty worthy of inclusion in a Paul Gauguin painting. She entrusts Paladin with a Signet Ring, a token from the King of the Sandwich Islands demonstrating a willingness to have his country made a U.S. Protectorate - and for which her avaricious step brother is prepared to kill her. After she leaves, the step-brother (costumed like an extra from "The Student Prince") arrives and insists she intends the ring as a gift for her Lover. But before he can introduce Paladin to this "Lover", he himself is fatally stabbed outside Paladin's suite. Searching for the murderer Paladin encounters a British Secret Agent who sleeps with his shoes on, as well as a burly, sinister German Professor of Military Science, former Naval Attache to the murder victim.
Before long there's another murder. Paladin's cunning trap eventually yields up the killer, but not before another weapon is added to a list of those brandished (or used) that already includes stiletto, skewer, sword, club, knife, cleaver and hat pin -- everything but a "blasted blow gun"!
Folks, this episode is just plain Fun. It isn't often you get to hear Richard Boone sing a snatch from Gilbert and Sullivan's "H.M.S. Pinafore". And the "Maltese" reference? Just a sly wink in the direction of the most famous McGuffin in all Moviedom!