Mr. Satterthwaite bumps into his old friend Mr. Quin at the opera at London's Covent Garden. During intermission, they spot a strikingly beautiful but distressed young woman in the audience. Later, they discover that she may be in real danger and decide to rescue her.
Librarian's note #1: this short story was published in Story-Teller (1927) and then in the print anthology The Mysterious Mr. Quin which was first published in 1930.
Librarian's note #2: the print anthology contains 12 Mr. Quin short stories. Besides those, there are two more found in other collections: The Love Detectives, and The Harlequin Tea Set. Mr. Satterthwaite also appears in Three Act Tragedy and Dead Man's Mirror. All by Agatha Christie, of course!
Librarian's note #3: the entries for all fourteen Mr. Quin short stories can be found on GR by searching for: a Harley Quin short story. Or for Harley Quin. Details such as characters and settings are included for each.
The Face of Helen - a Harley Quin Short Story by Agatha Christie - Bookist