This is the eighteenth book in the Wallace Boys series. 'The Sultan of the Sulu Sea' follows on from 'The Treasure of the Tiger' where the Wallace Boys and Singaporeans, Kheng Peng and Zainal, are aboard the Silver Spray after the traumatic hunt for General Yamashita's fabled treasure. The boys' return to Singapore is forestalled by the appearance of the pirate boat, the Sabil. Following a desperate attempt to elude the Moro pirates, the Silver Spray is captured and the boys are taken as hostages.The deliberate scuttling of the Silver Spray by the boys leads to tragedy, and the two Wallaces are transported to a small island in the Sulu Sea, the pirates' lair, from which rescue seems impossible.