TAS Season 1 Episode 14 The Eye of the Beholder

Plot

The USS Enterprise investigates the disappearance of a scientific team near Lactra VII. The starship Ariel is located there, abandoned, with its captain having transported to the planet's surface.
The Enterprise crew beams down to discover a series of unusual environments and are captured by the Lactrans, large sluglike beings with intellectual capacities far beyond their own. Science Officer Spock senses that they are telepathic but communicating at a speed too fast to comprehend. The team is installed in a zoo collection with the surviving crewmembers of the Ariel, one of whom is deathly ill. Enterprise Chief Medical Officer Dr. McCoy determines he could cure her if he had his medical kit, which has been confiscated along with their phasers and communicators. After the Ariel crew informs him that the Lactrans attempt to fulfill their captives' needs in response to impressions they pick up in their thoughts, Captain Kirk directs everyone to focus on the mental image of McCoy's medical kit. The Lactrans give them the kit and McCoy treats their comrade.
Kirk suggests similarly focusing on a communicator as an object they direly need. A young Lactran responds, and Kirk calls the Enterprise for a beam-up. The youngster snatches away the communicator, and is beamed aboard the ship in their place. The adult Lactrans, upset that their child has disappeared, focus their telepathic energy on Kirk, seeking an explanation. Due to the speed of their thoughts, this runs the risk of destroying Kirk's mind, so the other Starfleet officers project a mental barrier to protect Kirk.
Aboard the Enterprise, the youth probes Chief Engineer Scott's mind and processes the ship's entire library system. It proceeds to take the Enterprise galloping out of orbit. The child beams back down with Scotty. The youngster communicates what it has learned, and the adults decide that although still primitive, humans and Vulcans are in the process of evolving to a higher order, and are set free with a message that they will be welcome back in a number of centuries.