The secret adversary
(Book)

In 1919, young couple Tommy Beresford and Tuppence Cowley form a partnership, hiring themselves out as "young adventurers". Their first case, however, is more of an adventure than they expect--working to find documents that, if they were known to the general public, would fuel a communist revolution in Britain. Undercover agents Tommy and Tuppence know that Jane Finn was carrying top secret documents when she disappeared five years ago. What they don't know is that a killer is targeting a sinister older woman because she knows all about Jane. Soon Tommy and Tuppence are in grave danger.