Lord Trevor and Nan, his ward, England's famous diplomatic agents, receive word that there is something wrong going on in Egypt, but that the English government has been unable to learn anything tangible. Abdul, Trevor's faithful follower, is sent to investigate, and after six months had made considerable headway, but through an unfortunate discovery of his identity his usefulness is lost. Nan then determines to go in disguise as an Arab boy. So arrayed she runs a little fruit stand in the streets of Cairo. She succeeds in winning the confidence of the natives, and is taken in as a member of the conspiracy where she learns to her horror that the plan is to poison the food supply of the English and other foreigners by skillfully injecting germs of various deadly diseases. These germs are to be confided to water carriers, provision dealers and other humble servants, the idea being on a certain designated day to strike a deadly blow at all the English. Nan is able to foil the plot, but her identity is detected and she is in danger of losing her life, being only saved by the bravery and resourcefulness of the faithful Abdul. The two then destroy all the bottles of disease germs, and congratulate each other upon having foiled "The Mohammedan Conspiracy."