Hop Harrigan and his mechanic pal Tank Tinker are at a United Nations air base in the South Pacific, after rescuing an American secret service man, known only as Number 5, from a Japanese held island. The agent is in possession of the complete plans of a huge Japanese naval attack, and although badly beaten and in a semi-coma he managed to tell Hop, Tank and Major Waring that the plans are hidden in a false cavity in one of his teeth...