In this story, some soldiers go out drinking together and get drunk. One of the men, Fleete, does not know very much about India. On the way home he desecrates an Indian god, Hanuman the Monkey God, by stubbing out his cigar on the statue. A weird priest, who has the disease leprosy, grabs him and bites him on the chest, leaving a large and ugly mark. As the men laugh and leave the temple, another priest warns them that Hanuman will get his revenge.
Soon after, Fleete begins to act crazy. He is possessed. The monkey god has put a spell on him. His friend, Strickland, is familiar with the mark of the beast from the Bible (the number 666) and believes that if his friend does not get better, they will have to take drastic measures. Finally,
"Watch!" said Strickland. "If this happens six times I shall take the law into my own hands. I order you to help me."
Fleete's two friends kidnap the leper priest and force him to remove the spell on Fleete. Fleete recovers and has no memory of being possessed.
In the Bible, in the book of Revelation, the number 666 is called "the mark of the beast." It is the number of the Antichrist. Anyone that does not get this number stamped on his hand will not be able to survive. It symbolizes that one belongs to Satan, not God. When Fleete is bitten by the leper priest, he becomes a beast himself. He belongs to the beast.
Now -- what do you think the story means? I will give you a hint: it is an allegory of British colonialism in India. Think about this as you figure out the theme of the story.