She joined the crew of the USS Enterprise during a stopover at the Aldebaran colony, some time in 2265. Her assignment was to study crew reaction in emergency conditions.
She was aboard during Enterprise's first fateful excursion beyond the galactic barrier, a journey that changed Gary Mitchell forever. It changed her, too, but this did not become apparent until much later, at Delta Vega. There, she began to develop psionic energy abilities like those Mitchell had displayed.
Dehner had been aware of her esper rating (089), aperception quotient (20/100), Duke-Heidelburg quotient (256), and general knowledge quotient (654895-109) being better than average in all categories. This was evidenced since childhood in superiority at guessing games, reading cards, and other activities. This was, in part, the basis for her vocational training as a psychiatrist. Participation in tests and studies of other related psionic-oriented beings were the subject of a thesis published by her in association with the College of Medical Sciences of the Tri-Planetary Academy and was, in fact, the reason for her posting to the Aldebaran colony.
When Mitchell fled captivity, he took Dehner with him into the hills of Delta Vega. Captain Kirk followed, determined to put an end to the threat posed by Gary Mitchell. Sensing Kirk's pursuit, Mitchell sent Dehner to intercept him; he hoped she would learn just how insignificant mere mortals were. But she retained enough of her Humanity to be appalled by Mitchell's actions. She sacrificed her life to weaken Mitchell, an act that allowed Kirk to kill him.Kirk noted in his log that both officers died in the line of duty, since neither had asked for what happened.