In 2268 the starship entered a mysterious nebula, and Isel awoke to find herself pregnant. Three days later, Bones (Dr. McCoy) notes in his log that instead of undergoing the normal Deltan 10-month gestation period, the ensign is ready to give birth.
The birth goes well for Isel, who as a Deltan feels more pleasure than pain during the procedure. The child is found to be completely human with no Deltan DNA. The ensign then names the child “Irska” after her “father” since the word means “pure light” in the Deltan language. At the same time, a large cylindrical object approaches the ship, and Spock states that the entity has no machinery, but it does contain “dark light,” the same kind of energy the sensors detected the night Isel was impregnated.
The cylinder follows the Enterprise for a week, during which Irska not only survives, but grows into the body of a 10-year-old child.
By then, Isel is healthy enough to resume her duties, and the alien entity then transports a bit of dark light into the ship, enough to kill everyone in the next six hours.
As the child rapidly grows and ages over the next week the cylinder follows the ship and begins attacking unprovoked. Lt. Xon eventually determines that each of the attacks on the ship was meant to teach Irska something about life, death and emotions.The child realizes that her body is an “unnecessary shell”, and Isel takes her to the Transporter Room, where she beams her daughter into the cylinder, which then disappears as the Enterprise hull returns to normal.
After stating that she was the child’s first womb and the starship was her second, Isel states that her daughter had to learn what it was like to have a body before advancing into a state of pure energy.
Just then, a glowing ball appears and establishes contact with the Deltan before leaving the ship to explore the universe on her own.