"Return to Tomorrow" is a second season episode of the American science-fiction television series, Star Trek, first broadcast February 9, 1968, and repeated August 2, 1968. It is episode #49, production #51, written by John T. Dugan, under the pen-name "John Kingsbridge", and directed by Ralph Senensky.

In the episode, telepathic aliens take control of Captain Kirk, First Officer Spock, and Dr. Ann Mulhall's bodies.

The starship Enterprise receives a distress call from a lifeless planet. Upon arrival, a telepathic being named Sargon addresses Kirk and Spock as "my children", and insists they beam down to the planet. Sargon is very specific about the composition of the landing party, disabling the ship's power when Kirk refuses to bring Spock, and giving Dr. Ann Mulhall falsified "orders" to report to the transporter room. The destination coordinates are beneath 100 miles of solid rock. Kirk dismisses concerns about the safety of the party, asserting that if Sargon had wanted to kill them he could have already done so.

Kirk, Spock, Mulhall, and McCoy beam to a subterranean vault; Sargon prevents their armed accompaniment from dematerializing. Sargon greets them as a metallic sphere on a pedestal. He explains that he and two other spheres are the last survivors of their race. Their essences, stored in these metal orbs, have existed here since their planet was devastated by war 600,000 years ago. They once had physical bodies, but evolved into beings of pure energy. Kirk asks why Sargon refers to them as "children", and Sargon explains that the races of the Federation may be the descendants of his people, who colonized the galaxy long before their planet succumbed to war.

Sargon transfers his mind into Kirk's body and Kirk's mind into the sphere. McCoy realizes that Kirk's metabolism and temperature are rising to dangerous levels. Sargon explains that he will need the body only temporarily, long enough for himself and his companions to construct artificial bodies for themselves. He says he will need Spock and Mulhall's bodies for his companions. Concerned by his effect on Kirk's body, Sargon returns to his orb and restores Kirk. Kirk understands what Sargon wants and is not afraid of the potential effects. Sargon allows them to consider whether they will agree to Sargon's request.

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