Diaz was with Blake from the start in their pirate adventure which began with a conspiracy to steal a ship. This they did by setting up a business as a ship-broker and chartering a ship, Cygnet, under false pretenses. There followed a number of voyages where the ship was renamed several times, putting into one port, taking on cargo, absconding with it and selling the cargo at another port under a new name (see details in the article under John Sebastian Blake). Diaz's ship was finally beached on the north coast of Papua New Guinea during a typhoon while carrying a cargo of bar gold from Brisbane.
Blake and Diaz discharged the crew but remained behind to bury the gold somewhere near the wreck of their ship. They then stayed on for a while in a village of Kobe tribesmen. During this time, Diaz killed Blake in the course of a violent scuffle. Diaz then fled the village and took refuge in a neighbouring village of the rival Gilkik tribe. Subsequently, Diaz led a number of Gilkik raids on the Kobe village, presumably because he wanted to regain access to his buried gold. Unfortunately for Diaz, Biggles and co. had been assigned to investigate Blake and managed to locate the wreck of his ship. They were therefore on the scene and assisted in the the defence of the Kobe village during the last of Diaz's raids. The attacking Gilkiks were soundly defeated. During the retreat, Diaz was shot in the back by a poisoned arrow. Knowing he was dying, he confessed the location of his gold to Father Antoinne, a French missionary priest who worked among the Kobes, thus allowing Biggles to recover the gold and close the case.