Whilst spending a holiday in a small Central American Republic, Desmond Okewood, of the Secret Service, learns from a dying beachcomber of a hidden treasure.

With the assistance of a millionaire, he sets out for Cock Island, in the Pacific. To his astonishment he discovers that the Man with the Clubfoot, whom he had regarded as dead, has anticipated him. It is obvious to Okewood that his old enemy is also in search of the hidden gold, and there ensues a thrilling sequence of adventures, in which the millionaire's pretty daughter takes a prominent part.

Okewood has the cipher, and the Man with the Clubfoot determines to secure it, for without that cipher it is impossible to discover the hiding-place of the treasure; but there is something that the Man with the Clubfoot does not know, whereas Okewood does.