Dagger of the Mind

"Dagger of the Mind" is the ninth episode of the first season of the American science fiction television series Star Trek. Written by Shimon Wincelberg (under the pen name "S. Bar-David") and directed by Vincent McEveety, it first aired on November 3, 1966.
In the episode, the Enterprise visits a rehabilitation facility for the criminally insane where the chief doctor has been using a device which destroys the human mind.
This episode introduces the Vulcan mind meld.
The title is taken from a soliloquy by the title character in William Shakespeare's play Macbeth.
The USS Enterprise, commanded by Captain Kirk, makes a supply run to planet Tantalus V, a colony where the criminally insane are confined for treatment. The facility's director is Dr. Tristan Adams, a psychiatrist famous for advocating more humane treatment of such patients. After the Enterprise delivers supplies and receives cargo from Tantalus, a man emerges from the container taken aboard and assaults a technician. Reaching the bridge, the intruder demands asylum, but Spock subdues him with a Vulcan nerve pinch. In sickbay, the intruder identifies himself as Simon van Gelder, and a computer check reveals that he is not a patient, but Dr. Adams's assistant.
When they inform Tantalus of van Gelder's capture, Dr. Adams claims that van Gelder's testing of an experimental treatment device on himself is responsible for his disturbed condition. McCoy, suspicious, urges Kirk to investigate. Kirk transports down to the colony with one of the ship's psychiatrists, Dr. Helen Noel.
Adams introduces them to a strangely emotionless therapist, Lethe, and gives Kirk and Noel a tour of the colony. Although he is affable and accommodating, his staff, like Lethe, all seem lacking in affect. Adams shows Kirk and Noel the treatment device he referred to: a "neural neutralizer". He claims that the machine, harmless at low intensity, is used only to calm agitated inmates. Noel is satisfied with his explanation, but Kirk remains suspicious.
On the Enterprise van Gelder becomes increasingly frantic, warning that the landing party is in danger, but when he tries to explain the danger and refers to the neural neutralizer, he is convulsed with pain. Spock mind-melds with van Gelder to enable him to tell his story. Spock learns that the neural neutralizer can empty a mind of thoughts, leaving only an unbearable feeling of loneliness, and that Adams has been using it on inmates and staff to gain total control of their minds. The first officer assembles a security team, but the colony's force field blocks transport and communication.
Unaware of events on the ship, Kirk decides to secretly test the neutralizer on himself, with Noel at the controls. She finds that she can easily implant thoughts into Kirk's mind, even altering his memory of a recent Christmas-party encounter between the two of them. Adams appears, overpowers Noel, seizes the controls, increases the neutralizer's intensity, and proceeds to convince Kirk that he has been madly in love with Noel for years. Kirk and Noel are then confined to quarters.
On Kirk's orders, Noel enters the facility's physical plant through a ventilation duct, and interrupts Kirk's next neutralizer session by shutting off power to the entire complex. Freed from the neutralizer, Kirk attacks Adams, leaving him alone and unconscious in the treatment room. A guard discovers Noel's sabotage, they fight and she defeats him with an athletic kick that sends him hurtling into the electric circuitry, killing him. With the force field now off, Spock beams down to the planet, disables the force field, and restores power to the colony. This reactivates the neural neutralizer, and without anyone else in the room to say anything, it completely empties Adams' mind, killing him.
Back on the Enterprise, Kirk is informed that van Gelder has destroyed the neural neutralizer. McCoy is surprised that loneliness could be lethal, but Kirk, after his experience, is not.
The episode's title was taken from William Shakespeare's Macbeth. Macbeth is planning to murder the King of Scotland, and sees a dagger that he attempts to grasp, but it's only a hallucination.
Shimon Wincelberg originally wrote a reference to Hillel the Elder's “Torah on one leg” parable, but Roddenberry mandated an attribution to “the ancient skeptic.” Wincelberg, incensed by Roddenberry's rewrites, requested a name change to S. Bar-David for the airing.
Playing mentally disturbed Van Gelder took a toll on the actor Morgan Woodward. Reportedly when he finished filming the episode he went home and took a rest for four days.
In 2010, SciFiNow ranked this the eighth best episode of the original series.
Zack Handlen of The A.V. Club gave the episode a "B" rating, noting that the episode had "a handful of excellent moments (the mind-meld, that damn booth) that don't fit as well as they should.” Handlen noted Kirk and Noel's relationship as the plot's "weakest element,” and that Adams did not make a compelling villain. On the other hand, he felt that Nimoy made Spock's mind meld sequence "fairly effective.” The booth and its effect on Adams were also cited as memorable moments in the episode.