The Halkans are a people deeply committed to peace. Kirk is trying to negotiate with them for dilithium, the rare mineral that's needed for warp power systems. But if there is even a chance their crystals will be used for violence, they cannot agree. Kirk beams back to his ship as a magnetic storm approaches - but the ship where he materializes is vastly different, run by savage opportunists, sadists, and killers. This ship, part of an Empire, will wipe the Halkans from their world and seize their crystals. Kirk must find a way to prevent that, and to return himself and his landing party home.

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.