"Dagger of the Mind" is a first season episode of the science fiction television series, Star Trek. It is episode #9, production #11 and was broadcast November 3, 1966. It was written by Shimon Wincelberg under the pen name "S. Bar-David," and directed by Vincent McEveety. The title is taken from a soliloquy by the title character in William Shakespeare's play Macbeth.

In the plot, the Enterprise visits a planet that houses a rehabilitation facility for the criminally insane where a new treatment has horrifying results. It marks the first appearance of the Vulcan mind meld.

The Enterprise, commanded by James T. Kirk, makes a supply run to planet Tantalus V, a colony where the criminally insane are confined for treatment. The Tantalus 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 one of the containers taken aboard and assaults a technician. When they notify Tantalus of the escapee, First Officer Spock and Chief Medical Officer Dr. McCoy are surprised to learn that the stowaway is not a patient but Dr. Simon van Gelder, formerly Dr. Adams' assistant.

Van Gelder reaches the bridge, phaser in hand, and demands asylum. Spock subdues him with a Vulcan nerve pinch. McCoy, suspicious, wants to examine van Gelder and urges Kirk to investigate. Kirk transports down to the colony with the ship's psychiatrist, Dr. Helen Noel.

Adams introduces them to a strangely emotionless staff member, Lethe, and gives Kirk and Noel a tour of the colony. Although he is affable and accommodating, his staff, like Lethe, all seem remarkably wooden and lacking in affect, as though their emotions have been eliminated. Adams shows Kirk and Noel a treatment device which he claims is responsible for van Gelder's disturbed condition: an experimental neural neutralizer. According to Adams, van Gelder tested the device on himself, with horrifying results. 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.

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