Port Eternity is a romantic thriller set aboard the space yacht Maid of Astolat, owned by an extremely wealthy woman. She, her current lover and her cloned Azi servants live out her fantasy, each representing a figure from the world of Arthurian legend. However, disaster strikes when the ship hits a knot in space-time and they are drawn into a parallel dimension. There they gravitate to a huge mass consisting of ships of various species which had been similarly trapped in the past. With no way back, they join the cobbled-together cosmopolitan society.
The story takes place in the Alliance-Union universe, in Union-side space. Cherryh originally named the story Involutions, because it "spirals in upon itself", but after discussing the title with her publisher, Wollheim's suggestion of Port Eternity was adopted.
Although the book is a work of science fiction, some might consider it to be a science fantasy because of the Arthurian elements, medieval dress, and sometimes archaic mannerisms of the ship's inhabitants. Cherryh begins each chapter of the novel with a quote from Alfred Lord Tennyson's series of Arthurian poems Idylls of the King.