 On Dr. McCoy s orders, Mr. Scott joins him and Captain Kirk on shore-leave on Argelius, where the natives, especially the women, are extremely accommodating. Suddenly they find Scotty shivering with a knife in his hand used to stab repeatedly Kara, a girl whose corps lies at his feet. The local prefect and Mr. Hengist, a foreigner employed as administrator, lead the murder investigation. The Enterprise beams down a psycho-tricorder to probe Scotty s mind, but the murder knife to stir the prefects empathic wife Sybo is found in the chamber where Scotty was to be examined, in his hand- with crew member Lt. Karen Tracy similarly stabbed to death at his feet. The empath senses an ancient devouring evil, but the session is ended by a knife killing her. The investigation beams up to the Enterprise, where the best equipment and computer records produce a surprising lead...