Sent to investigate the destruction of several planetary systems, the U.S.S. Enterprise discovers the U.S.S. Constellation adrift in space. The only survivor, Commodore Matt Decker, informs the crew that a giant planet-eating machine made by a long-dead alien race is roaming the galaxy, consuming all in its path for fuel, including whole planets. Trapped on the derelict Constellation, Kirk can only stand by and watch as Decker takes command of the Enterprise, bent on seeking revenge against the planet killer.

William Windom (Commodore Matthew Decker)
James Doohan (Lt. Cmdr. Montgomery Scott)
George Takei (Lt. Hikaru Sulu)
Elizabeth Rogers (Lt. Palmer)
John Winston (Lt. Kyle)
Richard Compton (Lt. Washburn)
John Copage (Crewman Elliot)
Tim Burns (Crewman Russ)
Jerry Catron (Ensign Montgomery)

• The U.S.S. Constellation NCC-1017 is the first Federation starship to appear on screen, other than the Enterprise. (The ship model was a plastic toy model kit by AMT, which looked very cheap when the ship was entering the planet killer.)
  
• A shuttle launch is seen for the first time in this episode.
  
• Uhura does not appear in this episode; Lt. Palmer takes her place at communications.
  
• A preview of the new Main Engineering set is seen as the U.S.S. Constellation's engineering set. This new set will be seen in "I, Mudd". (Curiously, after Scott gets the Constellation running, he is seen thrown around on the old Enterprise Engineering set.)
  
• McCoy states that he is a "doctor, not a mechanic."