In a Trek murder mystery of galactic proportions, a series of killings on the hedonistic world Argelius II puts Scotty in the middle when he becomes the prime suspect. Worse yet, he can't even remember if he was responsible for the killings or not—he suffers from blackouts and memory loss at the instant of two of the three killings. As for the third murder ... he's certain it wasn't him—and had noticed a strange presence in the room after, as they say, the lights went out. The search for the truth eventually heads back to the Enterprise, where effective use of lie-detector equipment and extensive computer databases eventually leads to the uncovering of another suspect: the Argelian administrator Hengist (John Fledler).
This episode makes for a solid, interesting murder investigation with a few neat twists—including the revelation that the murderer is an alien entity that has jumped from body to body and planet to planet for centuries in its quest to feed upon other people's terror. At one point, the story explains, this presence even manifested itself as Earth's Jack the Ripper.
Although the plot is a little on the fantastic side, and Scotty's blackouts are never explained, the story pulls itself together nicely as Kirk and Spock find the vital clues in the database. I must admit, however, that the light-and-chummy ending seems a little out of place in an episode where confronting "ultimate evil" is a major theme.