The story begins in an antique shop owned by Wang Yun, where he shares with Steve Harrison the legend of the White Fox Spirit. Wang Yun is in the middle of feeding his pet cobra, Pan Chau, when he receives a mysterious phone call instructing him to stay by the machine until someone arrives. However, no one comes, and after hearing someone enter and quickly leave the shop, Wang finds nothing stolen but soon suffers a fatal cobra bite.

Steve Harrison, upon witnessing the incident, is skeptical of the circumstances, particularly noting that the cobra that bit Wang does not appear to be his usual pet, Pan Chau, but a younger, more vibrant snake. Harrison begins to investigate, suspecting foul play and a deeper mystery involving other characters connected to the story.

His investigation leads him to William D. Feodor, a scientist who had lost a cobra and was conducting experiments on snake venom. Feodor reveals that his cobra was stolen, and details emerge that suggest a link between his missing snake and the one that killed Wang Yun.

Harrison discovers that the motive behind the murder was a rare black pearl, known as The Black Moon, hidden inside one of Wang Yun's antiques. The detective sets a trap to catch the killer, using the pearl as bait. In a dramatic conclusion, Harrison catches Feodor red-handed, disguised as a Chinaman, attempting to steal the pearl. The story ends with the revelation that Feodor was color blind, which led to a crucial oversight in his plan.