Dagger of the Mind (Episode)
| Stardate | 2715.1 |
| Series | Star Trek |
| Episode Number | 10 |
| Release Date | 3 November 1966 |
| Writer | Shimon Wincelberg (as S. Bar-David) |
| Director | Vincent McEveety |
| Captain James T. Kirk | William Shatner |
| Lieutenant Commander Spock | Leonard Nimoy |
| Dr. Leonard "Bones" McCoy | DeForest Kelley |
| Lieutenant Commander Montgomery "Scotty" Scott | James Doohan |
| Lieutenant Hikaru Sulu | George Takei |
| Lieutenant Nyota Uhura | Nichelle Nichols |
| Ensign Berkeley | Larry Anthony |
| Dr. Tristan Adams | James Gregory |
| Dr. Simon van Gelder | Morgan Woodward |
| Dr. Helen Noel | Marianna Hill |
| Lethe | Suzanne Wasson |
A new treatment for the criminally insane has deadly results.
The Enterprise is delivering supplies to Tantalus V, a "progressive" penal colony directed by Dr. Tristan Adams. A Tantalus inmate escapes to the ship and demands asylum. An apparent raving madman, the patient is subdued and taken to sickbay. There, he is identified as Dr. Simon van Gelder, Dr. Adams' assistant.
Captain Kirk and ship's psychiatrist Dr. Helen Noel beam down to Tantalus to ensure that all is well. After they leave, Mr. Spock uses a Mind meld on van Gelder and determines that Adams has turned Tantalus into a chamber of horrors, using a Neural neutralizer—a device responsible for van Gelder's incoherent state.
Kirk and Noel experience the device firsthand when Adams uses it to convince Kirk that he is hopelessly in love with Dr. Noel. She risks her life to cut the colony's power so that Spock and a landing party can beam down through the planet's defensive force field. During the attack, Adams accidentally dies in his own machine, and van Gelder becomes the new director of Tantalus, where his first act is to dismantle the neural neutralizer.
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