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The starship Enterprise finds itself in conflict with an insubstantial enemy in this episode from the third season of the 1960s science fiction television series. The Enterprise is on a mission to deliver crew member Lt. Mira Romaine to a new post on Memory Alpha, a planet that serves as the central location of the Federation's massive, computerized library. Along the way, however, they encounter an unusual, glowing cloud of energy which attacks the ship, causing mysterious damage to the Enterprise's computers and several crew members. The Enterprise is still recovering when the cloud surrounds Memory Alpha, threatening the planet's inhabitants and libraries. The ship seems helpless to act, until the strange behavior of Lt. Romaine leads Dr. McCoy to suspect that she has developed some sort of mental connection with the energy cloud -- which may in fact be a living being. It is up to Captain Kirk to discover a way to exploit this link and figure out how to protect the Enterprise from further attacks.