The Thasians were a race of non-corporeal beings with extremely powerful psychokinetic abilities, native to the planet Thasus. They were once corporeal, being humanoid in form, but they eventually evolved into their present form. Once believed to be only a myth, Thasians were finally encountered by the USS Enterprise in 2266.
In 2252, a ship crashed on Thasus, with a young boy, Charles Evans, the only survivor. The Thasians found Charlie and granted him their psychokinetic powers so that he could survive. In 2266, Charlie was rescued from Thasus and transferred aboard the Enterprise from the survey vessel Antares for transportation to Colony 5, where he could be reunited with his closest living relatives.
The Thasians did not realize Charlie was gone until it was too late, however. Charlie's great psychokinetic powers, combined with his immaturity and petulance, made him a great threat to others he encountered. He took control of the Enterprise and even destroyed the Antares before the Thasians could intervene. Eventually, the Thasians did arrive, and were able to reverse all of the changes Charlie made aboard the Enterprise, but were unable to save the Antares. Taking on his form of centuries previous, before the Thasians had evolved beyond the need for physical bodies, a Thasian appeared on the Enterprise bridge as a translucent, floating humanoid head. He apologized to the crew of the Enterprise for the havoc Charlie had wrought on board, and for the destruction of the Antares, explaining that they had arrived to retrieve Charlie. When Captain Kirk asserted that Charlie should remain with his own kind, the Thasian countered by stating that Charlie was too much of a danger, and that he could never be taught to cease the use of his powers. Reluctantly agreeing, Captain Kirk allowed the Thasians to take Charlie back to Thasus, despite Charlie's protestations. (TOS: "Charlie X")
The alien character who appears at the end of "Charlie X" was originally to have been an elderly woman. In a memo that Associate Producer Robert Justman wrote Producer Gene Roddenberry (on 15 April 1966, while the episode had the working title "Charlie Is God"), Justman commented, "Insofar as the old woman goes, I would like to get into that with you a little further, because I believe that this character should be the boy's father." As of 22 April 1966, the woman was intended to be the mother of the boy Charlie, as evidenced by a letter from NBC Manager of Film Programming Stanley Robertson to Roddenberry. In an initial story outline of "Charlie X", Charlie's mother was referred to as a "Ferndok", with Charlie being half Ferndok and half Human. Also, at the conclusion of the story, Charlie was to make himself literally vanish, rather than any aliens taking him away.
In a revised story outline of "Charlie X", the alien Thasians were named in the plot. Concerning the member of the species who appeared at the end of the episode, Robert Justman, in a memo he wrote John D.F. Black (on 11 May 1966), wondered, "Would it be more believable if the Thasian's image materialized on the large Bridge Viewing Screen instead [of on the bridge itself]?"
When the first draft script of "Charlie X" was issued, however, the Thasian was still depicted as materializing on the Enterprise's bridge. In a memo from Robert Justman to Gene Roddenberry about that script (the memo was dated 7 June 1966), Justman considered, "Does the Thasian have to be vaguely humanoid in form? Why not this time can't we go completely far out and have nothing that resembles anything humanoid?" Justman, evidently saddened by the Thasian taking Charlie Evans away at the episode's conclusion, also questioned if it would be possible for the Thasian to "arrange things," so that they didn't need to take Charlie away with them but instead stripped him of the abilities they had given him.
In the final draft script of "Charlie X", the appearance of the Thasian was noted as involving "special effects" and was further described thus; "Something begins to materialize on the bridge. As it comes into complete view, we see it has a form, depth and width and height, but no solidity. It seems to waver and half-change (perhaps a flow of colors through it) as though it cannot hold this state very long. And it speaks in their [i.e. the Humans' and Spock's] tongue, but with a strange eerie VOICE that is in no way Human." Also, the Thasian's dialogue omitted the explanation that he was assuming his form from centuries ago. He had several ultimately unused lines of dialogue in the script. For example, he was very apologetic to Kirk for Charlie's behavior, taking blame for it on behalf of the Thasians in general, and mentioned that they had searched a long time to find Charlie. In addition, the alien assured the Humans, "As soon as we were close enough, we took him in control."
The Thasian was depicted using an ethereal rippling effect that was added by the Howard Anderson Company. The same effect was also used to represent the Balok puppet in TOS: "The Corbomite Maneuver". (The Star Trek Compendium, 4th ed., p. 33)