• Production Code: 73
• Stardate: 5725.3
• Original Air Date: Jan 31, 1969
• Writer: Jeremy Tarcher and Shari Lewis
• Director: Herb Kenwith
• Guest Stars:
  • Jan Shutan (Lt. Mira Romaine)
  • Majel Barrett (Nurse Chapel)
  • John Winston (Lt. Kyle)
  • Libby Erwin (Technician)

En route to Memory Alpha, the home of the Federation's largest library/computer banks, the Enterprise is transferring Lt. Romaine to her next assignment, overseeing refits and new installations on Memory Alpha. A cloud of energy intercepts the ship and wreaks havoc with the Enterprise's instruments and crew, affecting various crewmembers' brains in different ways and causing Lt. Romaine to pass out. The cloud strikes Memory Alpha next, wiping out every living thing on the planetoid along with most of the library banks. Mira, who has been experiencing strange thoughts and visions since the cloud's first sweep of the Enterprise, is suddenly able to predict the cloud is returning to the vicinity before the Enterprise's sensors can. Kirk orders phasers fired to defend the ship, but every time the cloud is hit, it injures Lt. Romaine. McCoy determines that the energy beings in the cloud are now telepathically linked to her mind.