Piper was present on the bridge when the Enterprise crew embarked on an exploratory expedition outside of the Milky Way Galaxy. After the ship's encounter with an exotic energy field at the galaxy's edge, Piper tended to Lieutenant Gary Mitchell and Doctor Elizabeth Dehner, who were mysteriously affected by the barrier. Later, Piper accompanied Captain Kirk and a landing party to the surface of Delta Vega, where the crew was attempting to adapt the power packs of an automated lithium cracking station to regenerate the ship's warp engines. While there, Piper observed Lieutenant Mitchell, who had begun to manifest extremely powerful psychokinetic abilities as a result of his exposure to the energy barrier. Along with the rest of the landing party, Piper was knocked unconscious by Mitchell as he escaped with Dehner. After regaining consciousness, he revived Captain Kirk, who departed after Mitchell, but not before imparting orders to Piper that the entire planet be subjected to a lethal dose of neutron radiation, should Kirk not report back within twelve hours.