This is the sixth book in the popular Frank Herbert series centering on the desert planet Dune/Arrakis, or Rakis, as it is known in this novel.
Tyrants and other mythic religious figures may come and go; the market for melange (a life extending drug) may fluctuate and the worms still turn the sands of Dune, but life still goes on--and on--in the Million Planets.
This novel focuses on the Bene Gesserit, a Sisterhood that has helped to manipulate and sustain events in the civilized sectors of the universe. The Bene Gesserit's home planet is called Chapterhouse. This is where they train novitiates, continue their search for balance out of chaos, and during the course of the story, chart the destruction of the current evil menacing and destroying planets wholesale.
The nasties are a rather twisted mutation of the Bene Gesserit called the Honored Matres, whose main weapon is ultimate sexual expertise that enslaves the men they come in contact with.
We've also got gholas (clones, of a sort), Tleilaxu Masters, axlotl tanks (where gholas are developed) no-ships, nullentropy tubes and a host of other artifacts and folks, all bound up with the clever and subtle machinations of the Bene Gesserit as they tweak and prod history. They extend even further the multifaceted universe of Herbert's Dune but don't add up to much here.
We also are stuck with a pretty dull story that hasn't nearly the bite and pacing of the previous volume, "The Heretics of Dune."
One of the focal characters, Darwi Odrade, now the Mother Superior of the Bene Gesserit, constantly agonizes internally and with her fellow Reverend Mothers over her decisions, over past actions and their ramifications and the future of galactic civilization as they know it. This makes interesting midnight philosophy discussions but slow reading.
Duncan Idaho makes a come-back, as does the Bashar , Miles Teg. Duncan has a passionate love, a won-over, ex-Honored Matre named Murbella, and they're isolated on a huge no-ship so as to elude discovery by any unfriendliness with an exiled Tleilaxu Master named Scytale, the last of his kind.
Herbert leads you on as to the importance of this character and what he might do, but nothing really materializes. By the end, his existence is a footnote. And so are the rest of the subplots, almost all of which are unfinalized and their position in the story not particularly important. All the action takes place at the conclusion, which lacks wallop. At this stage of the series, we're beating an already extremely attenuated idea. If this is set up because there is to be yet a seventh book, I won't hold my breath.