James T. Kirk/Starfleet career/USS Enterprise/Year One

For five years, Kirk commanded the Enterprise, which made him a legend in space exploration. (; ) In addition to his primary mission statement – "to explore strange new worlds, to seek out new life, and new civilizations" – Kirk received standing orders to investigate all quasars and quasar-like phenomena. ()
On more than one occasion, Kirk and Mitchell took part in missions together, including one on Dimorus, where they encountered rodent-like creatures that shot poisonous darts. Mitchell took one of the darts meant for Kirk, saving Kirk's life but nearly dying himself. The two later visited Deneb IV where, in at least three cases, Mitchell was capable of carrying long telepathic conversations with the natives, scoring 80% or higher on comprehension. One night, a telepathic conversation with a female native had a deleterious effect on Mitchell. Kirk later stated that he'd been worried about Mitchell ever since that night. As a pun, Mitchell referred to the girl as a nova. ()
Following a stopover at the Aldebaron colony, the Enterprise attempted to determine the whereabouts of the missing . After discovering the Valiants disaster recorder which described a catastrophic disaster following that early vessel's visit to the galaxy's edge, Kirk pushed on and encountered the galactic barrier for his first time. The Enterprise failed to breach the barrier and barely escaped destruction. With its warp engines badly damaged, the Enterprise limped under impulse power towards the Delta Vega lithium cracking station.
The barrier triggered a transformation in Mitchell who began developing psychic powers that progressed rapidly, with a commensurate loss of his humanity. Ignoring Spock advising him to destroy Mitchell immediately, Kirk hesitated until after Mitchell killed navigator Lee Kelso. On the surface of Delta Vega, he hunted Mitchell and managed to kill him only with the help of another officer undergoing the same transformation as Mitchell, Dr. Elizabeth Dehner. ()
Following the tragedy, Kirk shuffled the Enterprises command crew. Lieutenant Commander Spock remained science officer and Kirk acknowledged him as first officer. A new chief medical officer, Dr. Leonard McCoy, replaced Dr. Mark Piper. Lieutenant Commander Montgomery Scott remained chief engineer. Lieutenant Nyota Uhura became communications officer and Lieutenant Hikaru Sulu was transferred from astrosciences to the helm. Kirk did not settle on a regular navigator for another two years. ()