Originally published in Intrigue Mystery Magazine #1, October 1965. Reprinted in two collections by Bill Pronzini and Martin H. Greenberg: 13 Short Espionage Novels (1985) and The Mammoth Book of Short Spy Novels (1986) [essentially the same collection but with one book removed]. Contained in the collection .
A scientist for a major new military weapon has been injured. I.C.E. wants his plans and kidnaps the male twin of a the man's mistress to force her to obtain it and hand it over. Naturally, she is also a princess and naturally Peter Baron will use his charms to dissuade her and to save the day.