The Enterprise is en route to planet Babel with Commodore Robert April to celebrate his retirement. An unusual spaceship pulls the Enterprise into a negative universe where time seems to flow backwards.

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Commander Spock)
DeForest Kelley (Dr. Leonard H. McCoy)

George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Majel Barrett (Nurse Christine Chapel)
James Doohan (Lt. Cmdr. Montgomery Scott)

• The series finale.
  
• Robert April is 75 years old, mandatory retirement age for Starfleet. He was the first commanding officer of the U.S.S. Enterprise.