The crew of the U.S.S. Enterprise is completing a treaty mission with the Maabas, a peaceful alien race not native to the star system they currently inhabit, as descendants of refugees from a great war long ago. Several hundred thousand Maabas once took shelter on their new world, and have now been here for mil­lennia. They do not travel the stars, but seek to explore from within. The Federation’s interest is in the Maabas’s great intellec­tual resources — their science, while behind in some areas, excels in others, and their philosophy is in line with that of the Federa­tion. But just as the pact is signed, the Enterprise is attacked by an unidentified vessel. Enough force is shown to keep the alien assailants at bay, but a new danger arises. Their mysterious foes are the Kenisians — a race that used to inhabit the Maabas’s chosen world thousands of years ago, and who now want to take it back.