The planet of a negative universe, where time flows backward, reached by the Enterprise in The Counter-Clock Incident.
Arret is a planet in a reversed universe where time runs backward, its name being Terra spelled in reverse. In The Counter-Clock Incident, the Enterprise follows a ship piloted by Karla Five through a supernova and into that universe, and Karla takes them to Arret.
On Arret, Karla's elderly son, Karl Four, helps the crew find a way back. Meanwhile the Enterprise crew are growing younger, and Robert April leads the escape.