The alien ship's pilot, Karla Five, is an explorer from the reverse universe who accidentally became lost in our universe. He leads the U.S.S. Enterprise to his home world of Arret to help find a way back into normal space. On Arret, the crew encounter an old man, who is Karla Five's son, and a baby, his father. The crew begin to notice that they too are getting younger as they remain in the negative universe.

In time they discover a dead star that corresponds to the nova in the positive universe — it is the gateway for the U.S.S. Enterprise's return. Though Karla Five's ship is strong enough to pull the starship through the nova, the Enterprise crew is now too young to pilot it. Commodore April, now a young handsome man, takes command of the ship steering it back through the nova. Using the transporter, April restores the crew to their proper age. The Aprils consider remaining young, but instead decide that they have lived their lives to the fullest. Once back to their original ages, the Federation decides to change the mandatory retirement age and the Aprils are able to continue their careers.