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Several of the Enterprise's top crew members, including Captain Kirk, are held captive by a madman in this episode from the third season of the 1960s science fiction series. The starship is entrusted with the shipment of supplies to a mental-hospital planet, including a new, powerful drug used to treat the dangerously insane. When Kirk, Mr. Spock, and several other crew members visit the planet's surface, however, they unwittingly stumble into a trap -- too late they discover that the man they believe to be the hospital's director is in fact a disguised patient. This patient, himself a former starship captain, activates the planet's security devices to keep the Enterprise crew captive, forcing Kirk and the others to attempt to outwit a brilliant madman.