The Secret Adversary -by Agatha Christie (first adventure of Tommy and Tuppence)

The case of "Jane Finn" brought series of curious and dangerous events in the life of the young adventurers, Mr. Thomas beresford or Tommy and Ms. Prudence Cowley or Tuppence. The only thing they knew about Jane Finn that she was missing for five years and carrying top secret documents. Two adventurers had to fight with the unknown and powerful enemy but the enemy knew where to find Tommy and Tuppence......