From Goodreads:
Set during the early days of World War II. Celia McNeill is detained for visa problems when traveling to a new job in Paris. Desperate for cash, she makes a deal with card master Sergei Radetzkoy, whom she meets while being held in Dieppe.
He says he's just lucky at cards, but the crooks who follow him from the casino aren't interested in his winnings. They demand to know his 'system'. The fashionable folk who rescue him don't conceal their interest in his extraordinary success at the casino either. This leads to kidnapping, betrayal, and murder under the looming threat of the Nazi invasion.