A psychiatrist, Dehner was assigned to the U.S.S. Enterprise in 2265 to study the crew's reactions to crisis situations. When the ship crossed the Galactic Barrier, the energy field there mutated Dehner into a god-like being, along with Lt. Commander Gary Mitchell. Dehner and Mitchell were later killed when Captain Kirk sought to quarantine them on planet Delta Vega. Both were later listed as having given their lives in the line of duty.