 A huge cosmic cloud is reported moving into the outer fringe of the galaxy. Starfleet Command sends the Enterprise to investigate. Sensors indicate it is a strange combination of Matter & Energy. It seems to engulf and feed on the energy of planets. It makes a deliberate course change to head towards the populated planet of Mantilles indicating that it could be intelligent. Kirk sees it as a threat to every planet in the galaxy and if they can t communicate with it and make it stop it will have to be destroyed.