After Buchan identifies the killer's modus operandi as aping the Marquis De Sade, who murdered women after poisoning them with an aphrodisiac, Chandler and Miles hunt for the individual who contacted a firm specializing in love potions by masquerading as a couple. This leads the team to S and M club the Hooded Crow, where Mina almost falls victim to a man with vampiric tendencies. Then two more victims are found, identical in height and appearance to the first two. Chandler overrules Mina and follows Miles' suggestion that they concentrate on finding Ella Bowe. This proves to be the key to the serial killer's identity.
—don @ minifie-1