Diana Muldaur (Dr. Pulaski in the second season of Star Trek: The Next Generation) guest stars on this fascinating episode of the Original Series as a scientist chosen to be the host for a female god-like lifeforce, pure incorporeal and without physical form (pure thought, pure energy), along with her "beloved", Sargon (the one whose voice from thought "spoke" to Kirk and the Enterprise, requesting his presence on their planet, inside a chamber located deep within rock, prepared before a cataclysm millions of years ago that happened on the surface. Three cylinder-shaped spheres encase the energy forms, Thalassa (to use Muldaur's Mulhall), Sargon (to use Kirk's body), and Sargon's former enemy, Henoch (to use Spock's body). They want to exist in the bodies of the humans they have chosen as they set out to create robotic bodies to house their forms, but Henoch is eyeing Thalassa for himself, loving the body of Spock, initiating a scheme to murder Kirk's body and hopefully rival, Sargon, as well. Nurse Chapel (Majel Barrett) is to help Henoch kill Kirk by using a Vulcan mind manipulation without her knowing what he has her doing.
This will probably best remembered for a couple of key scenes and performances. Nimoy did get chances to evoke emotion besides the cold and analytical Vulcan. "Return to Tomorrow" allowed him to play a scheming, mischievous, cold-blooded sociopath with a twinkle in his eye and a cat-that-ate-the-canary grin. Thalassa angered at Bones, insulted at his "not into peddling flesh" comment and defying her attempt to reason with him to keep the secret of her holding onto the body of Mulhall, causing him purposed pain to prove a point about how easy it'd be to harm him (that he should be on his knees worshipping at her feet). Henoch outsmarted by Sargon who he thinks is dead. The episode actually having Kirk dead (and later Spock), with Bones realizing this with startled astonishment. Shatner's performance as Kirk's body is taken over by Sargon for the first time, and that re-introduction to human flesh and blood providing great satisfaction, forgetting how great it feels. The love story between these god-like beings reliving yesterday at the present in these bodies which allows them to feel each other after so long. Kirk's speech about what Sargon could provide them in terms of its knowledge and superior intellect. Chapel (always smitten with Spock with her infatuation falling on deaf ears) getting to share her body with Spock's consciousness. All the chicanery by Henoch, often mocking Thalassa, as he plots to keep his Vulcan body and all its pleasantries. And on and on. It is an episode with lots going on, giving Shatner and Nimoy a real chance to go outside their comfort zones and do something different with their characters. That Kirk and Spock suffer technical deaths only to be spared thanks to the power of these beings which can inhabit even the Enterprise (!) produce some knockout moments.
This will probably best remembered for a couple of key scenes and performances. Nimoy did get chances to evoke emotion besides the cold and analytical Vulcan. "Return to Tomorrow" allowed him to play a scheming, mischievous, cold-blooded sociopath with a twinkle in his eye and a cat-that-ate-the-canary grin. Thalassa angered at Bones, insulted at his "not into peddling flesh" comment and defying her attempt to reason with him to keep the secret of her holding onto the body of Mulhall, causing him purposed pain to prove a point about how easy it'd be to harm him (that he should be on his knees worshipping at her feet). Henoch outsmarted by Sargon who he thinks is dead. The episode actually having Kirk dead (and later Spock), with Bones realizing this with startled astonishment. Shatner's performance as Kirk's body is taken over by Sargon for the first time, and that re-introduction to human flesh and blood providing great satisfaction, forgetting how great it feels. The love story between these god-like beings reliving yesterday at the present in these bodies which allows them to feel each other after so long. Kirk's speech about what Sargon could provide them in terms of its knowledge and superior intellect. Chapel (always smitten with Spock with her infatuation falling on deaf ears) getting to share her body with Spock's consciousness. All the chicanery by Henoch, often mocking Thalassa, as he plots to keep his Vulcan body and all its pleasantries. And on and on. It is an episode with lots going on, giving Shatner and Nimoy a real chance to go outside their comfort zones and do something different with their characters. That Kirk and Spock suffer technical deaths only to be spared thanks to the power of these beings which can inhabit even the Enterprise (!) produce some knockout moments.