Milverton blackmails Mary, exploiting her dalliance in antigovernment activities as a student. Sherlock decides to sneak into Milverton's estate to steal his evidence, but even that act was anticipated. Sherlock finds that he is not the only trespasser.
then as long as your
drama plays itself out,
haben Sie keinen Grund, Mary zu erpressen,
solange dieses Stück sein Ende findet.