A criminal known as the 'thistle killer' is on the loose in London. For several nights in a row, he has killed a young woman, and has left three thistles near the body. Dr. Watson thinks it is the work of a maniac, but Sherlock Holmes is convinced that there is a pattern. Holmes is not surprised when Inspector Lestrade arrives to consult him. Based on his study of the locations of the killings, Holmes already knows where the next attack will take place. Holmes, Watson, and Lestrade prepare a plan to try to catch the killer in the act.