"Charlie X" is an episode of the original Star Trek series aired on September 15, 1966, with the Star Date of 1535.2. The Enterprise has arrived at the system of Tarsus IV and is met with an unusual young man, Charlie Evans. Charlie is the sole survivor of a transport attack by the Thasians, a highly advanced race of aliens.

Captain Kirk, Spock, Dr. McCoy, Scotty, Sulu, Checkov, and Uhura all have individual roles in assisting Charlie, a young man struggling to understand the humanoid culture. In addition, three female actresses are featured in "Charlie X." The first is Juli Reding, playing the role of Ruth, a new girl Charlie falls in love with. The second is Sarah Marshall, who plays the Thasian emissary sent to take Charlie back with her species. The third is Shelly Novack, an unnamed guest with romantic feelings for Charlie.

The antagonists of this episode are the Thasians, who have given Charlie the power to control people's minds and emotions with a thought. With his newfound powers, Charlie causes havoc on the Enterprise, endangering the crew and passengers. The crew must work together to find a solution to the problem.

The USS Enterprise meets the vessel Antares to take custody of Charlie Evans, the only survivor of a transport ship that had a crash on the planet Thasus. Charlie, who was 17 years of age at the time, was stuck in the wreckage for fourteen years, picking up talking skills from the Ship's computer systems which were still functioning. He is to be brought to his closest family members who live in the Alpha V colony. Although the crew members of Antares had nice things to say about him, they were happy to see him go from their Ship. Charlie informs Dr. McCoy that the people on the Antares do not care for him, and he only desires people to like him. Despite his eagerness to please, Charlie is quite a nuisance due to his lack of social understanding and ability to regulate his emotions. He bonds with Captain Kirk and is attracted to Yeoman Janice Rand.

Charlie shows off unique telekinetic and material-shifting talents, yet the team needs to realize it. He runs into Rand in the fun room, where Mr. Spock plays a Vulcan harp, and Lt. Uhura starts singing. Charlie gets miffed that he's being used as the subject of Uhura's show and that Rand is showing more enthusiasm for the music than for him, so he makes Uhura lose her voice for a moment and spoils Spock's instrument.

When the Antares was on the verge of disappearing from the sensors, it attempted to warn the Enterprise, but the transmission was cut short. Scans indicated that the Antares had been utterly destroyed. Kirk attempted to show Charlie some martial arts moves, but Sam, Kirk's training partner, made fun of Charlie's blunder, and he made Sam mysteriously vanish. Shocked, Kirk commanded the security guards to take Charlie back to his cabin. Charlie made all the phasers on the Ship disappear but eventually obeyed Kirk's order to return to his room. Reports showed that Charlie had the same powers as the Thasians, yet the medical exam given by McCoy when Charlie first boarded the Ship confirmed he was just a human. Charlie admitted he used his abilities to remove a critical component from the Antares.

Feeling discontent with the newfound hostility between him and the team, Charlie escaped his quarters and used his abilities to manipulate the group, altering their appearance or halting them in their tracks as he desired. For example, when Rand denied his romantic pursuit, he made her vanish. When Kirk asked if Rand was still alive, Charlie refused to answer.

Once Kirk realized Charlie's abilities were too powerful to be restrained, he chose to deviate away from Alpha V to keep Charlie away from populated areas where he could cause destruction. Charlie uncovered Kirk's plan and seized the Enterprise. Kirk speculated that manipulating the spacecraft may deplete Charlie's power, so he requested all the Ship's systems to be enabled and launched an attack against Charlie. His theory was incorrect, but it did divert Charlie's attention from the pursuing Thasian Ship.

A ship from Thasus pulls up and fixes up the Enterprise and all its occupants, excluding the Antares. The Thasian commander reveals that his people gave Charlie special abilities to survive on their planet. Still, these abilities (which cannot be taken away from him) make him too risky to remain with humans. Charlie pleads with Kirk to not let the aliens take him since the Thasians have no physical form or capability for love. However, the Thasians dismiss Kirk's point that Charlie should be with his own kind and, with a last resounding cry of "I wanna stay!" Charlie beamed away, and Yeoman Rand started to sob.

Juli Reding, who played Ruth in this episode, went on to appear in other films and television shows such as "Bonanza," "The Streets of San Francisco," and "Fantasy Island." Sarah Marshall, who played the Thasian emissary, also appeared in "The Invaders" and "Ironside." Finally, Shelly Novack, the unnamed guest, can be found in "Bewitched" and "Green Acres."

This episode was filmed at Desilu Studios and is famous for being the first to show the crew in full color. It is also the first episode to feature a stunt double for the female characters.