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.

Mr. Norman is a new crewmember whom McCoy finds off-putting. He acts like a Vulcan, which is odd in a human. There's a good reason: Mr. Norman is not human. He is one of a collection of androids sent to hijack Enterprise by Kirk's old adversary, Harcourt Fenton Mudd! Mudd has plans for the starship, grand plans. And so do the androids.