Polina Serova unexpectedly for herself becomes a secret agent of the Russian emperor! With officer Alexey Kaverin, she arrives in Paris to carry out her first assignment—to obtain secret documents that are extremely important for Russia. Together, Alexey and Polina attend a masked ball at a mansion where a casket with the documents is hidden, but instead they find another casket—with some old letters… To avoid keeping evidence, Alexey gets rid of the unnecessary casket, but soon it turns out that those letters contain a route to treasures of the French crown that King Louis-Philippe himself is searching for! Now Polina and Alexey will have to find what they so rashly threw away. And who can help them? No one less than the most famous detective of all times—Vidoc!