Episode Title: "RETURN TO TOMORROW"
Production Number: 51
Original Air Date: 02-09-68
Hundreds of light-years beyond where any Earth ship has explored before, the U.S.S. Enterprise picks up a mysterious signal from a star system ahead, where the crew finds an ancient Class-M (Earth-like) planet whose atmosphere was ripped away about a half million years ago.
An alien entity named Sargon contacts the Enterprise from the lifeless planet, and gives transporter coordinates for an underground cavern. A landing party finds that Sargon is a spherical container full of pure energy -- the essence of Sargon's mind. Sargon's people once had humanoid bodies; six thousand centuries ago their ships colonized the Galaxy, before destroying themselves in a massive conflict.
Sargon takes over Captain Kirk's body with his mind, and says that he needs to borrow two other bodies for the two other remaining minds, so that they may construct android bodies for themselves after half a million years as disembodied minds. After the Enterprise officers agree, the two other entities, Henoch and Thalassa, enter the bodies of Commander Spock and astrobiologist Dr. Anne Mulhall.
As the three aliens work on building android bodies, Henoch wants to keep Spock's body for himself, and plots to kill Sargon in Kirk's body. Sargon and Thalassa realize that the temptation of their mental powers is too great, and they trick Henoch into leaving Spock's body while Spock's consciousness is placed in Nurse Chapel's mind. The aliens leave, with the officers restored to their correct bodies.
Factoid: From the current position of the U.S.S. Enterprise, a message will take over three weeks to reach Starfleet.
Dr. McCoy: "He's dead."
Dr. McCoy: "I will not peddle flesh. I'm a physician."
Guest Characters: Nurse Christine Chapel, *Dr. Anne Mulhall, Lt. Leslie
KEY: [brackets]=illusions *star=first appearance