Thalassa was one of three survivors of a species that died half a million years ago. She was the wife of Sargon. The two of them, along with their seemingly former enemy Henoch, housed their consciousnesses inside receptacles, which the USS Enterprise discovered in 2268. She would borrow the body of Ann Mulhall, an Enterprise crew member, in order to build androids that could house her mind.
After she, Sargon and Henoch had transferred to their borrowed bodies, she was tempted by Henoch to keep the body of Ann Mulhall. She offered to save Captain James T. Kirk, whose body, which housed Sargon, was poisoned by Henoch, if she could keep her Human form. After coming to her senses, she helped Sargon kill Henoch, and she and Sargon returned to their pure consciousness state and embarked from the Enterprise to oblivion. (TOS: "Return to Tomorrow")