(This has been called one of the two most famous riddle stories of all time, along with "The Lady, or the Tiger?" by Frank Stockton.)
Burwell, a New Yorker in Paris, during intermission at the Folies Bergere, is inconspicuously given a card by a charming, elegant lady passing by his table with a gentleman. The card bore some French words written in purple ink. Not knowing that language, he was unable to make out the meaning. He returned at once to his hotel to inquire concerning the message on the card, where he asked the hotel manager to translate the roughly 20 words. As he read, the manager's face grew rigid with astonishment. He exclaimed:
"Where did you get this, monsieur?"
Burwell started to explain, but was interrupted by: "That will do, that will do. You must leave the hotel."
"What do you mean?" asked Burwell, in amazement.
"You must leave the hotel, now--tonight--without fail!" commanded the manager, excitedly.
At the next hotel, Burwell received the same reaction. Relentlessly pursuing the solution to the mystery of the card, he suffers a succession of unfortunate experiences. The story ends without a clear resolution, leaving the readers to ponder the mystery.
The author revealed the solution to this puzzle in the sequel, "The Mysterious Card Unveiled," which he didn't publish until the next year, to keep his readers in suspense.
(The sequel will also be found at Feedbooks soon after the first part has been published. Don't cheat -- guess a solution to the mystery before reading the sequel!)