"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.