Biggles and Algy travel to Bonney Island, in the Indian Ocean, after reports that this deserted outpost of the Empire is not quite as deserted as believed.

On the island they find a reclusive Englishman called Collingwood, an Arab called Ali, and a field full of hashish. Jumping to the obvious conclusion re drug dealing, they destroy the drug, only for Collingwood to then warn them of the boatload of Arabs it belongs to. His reason for being on the island turns out to be Opals.

In a murderous finale, Ali murders Collingwood. Ali is then himself murdered by one of the Arab gang recently returned on their ship. A boatload of the crew are killed by a decapod (Capt Johns loves those decapods!) and then Ali's murderer is eaten by a shark.

All the while Collingwood's box of opals moves from man to man, just before they are killed. When it is finally recovered by the Arabs on their dhow, the boat is smashed on the rocks and the remaining gang-members left on the island as Biggles and Algy fly off to make their report.