From inside cover:

In the future, "jumpdoors" presented to mankind by the mysterious entities known as Calebans have made instantaneous travel -- to any point in the Galaxy -- a commonplace. But the Calebans are vanishing, and now only one is left -- and two agents of the Bureau of Sabotage make a horrifying discovery: When the last Caleban dies, so will everyone, everywhere, who has ever used a jumpdoor! And nobody is known *not* to have "jumped" at least once....

The Caleban *must* be kept alive -- but it is under contract to a psychotic millionairess who is subjecting it to systematic, damaging torture....

From this weird situation, Frank Herbert spins a colorful, fast-moving tale, set in an exotic future civilization -- deft, dramatic, shimmering with the "sense of wonder" that marks the best SF.