Darnell was a male Human Starfleet crewman who lived during the mid-23rd century. He served in the sciences division aboard the USS Enterprise under Captain James T. Kirk, during a historic five-year mission that starship undertook.
In 2266, Darnell accompanied Kirk and Doctor McCoy to the surface of planet M-113, for the purpose of performing routine medical checks on Robert and Nancy Crater. While there, all three members of the landing party saw a different version of Nancy Crater, who was, in reality, a creature able to read the minds of its prey and project whatever image of itself that it wanted. The woman Darnell saw was a recreation of a beautiful woman he had once met on Wrigley's pleasure planet. When he mentioned this to "Nancy", McCoy hushed him and Kirk then posted Darnell out of sight of her.
After successfully luring Darnell away, the creature killed him by extracting all of the salt from his body, leaving a series of red skin mottlings on his face where he had been attacked. The creature subsequently placed a borgia plant in his mouth, for the purpose of misdirecting the Enterprise crew into believing that he had died by consuming a poisonous substance. (TOS: "The Man Trap")