Enterprise discovers the wreck of Constellation, a sister ship. The only survivor is Matt Decker, the Captain. He reports beaming his crew down to the fourth planet - except that there is no fourth planet! He investigated an odd occurrence: a planet apparently disintegrating, and discovered an immense space vessel cutting the planet apart with an antiproton beam, and digesting the pieces! This machine came from outside the galaxy, a relic of a devastating war fought long ago and far away. It will carve a path of destroyed solar systems through the most populous part of the Milky Way unless Kirk and his crew can stop it.