A pilot from a universe where time runs backwards, whose ship draws the Enterprise through a nova in 2270 and whose aged son, Karl Four, helps the crew to return.
Karla Five appears in The Counter-Clock Incident, the final animated episode. The Enterprise meets her small ship travelling at an impossible speed towards the nova Beta Niobe. Kirk puts a tractor beam on it to save her, and both vessels are dragged through into another universe.
There space is white and the stars black, and people are born old and grow younger. Karla Five had fallen into the Federation's universe by accident when a star went nova at home.
She takes the crew to her planet, Arret, where her son Karl Four, an old man, and her father, an infant, work out that two novas occurring at once in both universes will open the way back. Commodore Robert April takes command as the crew grow younger. Her voice was provided by Nichelle Nichols.