The Enterprise encounters a giant cloud creature that feeds on the energy of the planets that lie in its path. They determine it is heading for Mantilles, home to a Federation colony governed by former Starfleet officer Bob Wesley.

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 (Cosmic Cloud)
James Doohan (Lt. Cmdr. Montgomery Scott)

• This is the first episode where Lieutenant Arex speaks (voiced by James Doohan).
  
• Governor Bob Wesley (voiced by James Doohan) previously appeared in "The Ultimate Computer", where he commanded the U.S.S. Lexington. (He was portrayed by Barry Russo in the live action episode.)