Farmer tells the tale of one person whose is uniquely suited to find the river's headwaters, riverboat captain and famous Earthly author Sam Clemens (a.k.a. Mark Twain). Clemens has been visited by "X," a mysterious being who claims to be a rebel among the group that created Riverworld. X tells Clemens where he can find a large deposit of iron and other materials that Clemens can use to build the greatest riverboat ever seen. Since there is virtually no metal on the planet, it will also give Clemens an unbeatable edge when it comes to battling the various warlike societies that dominate the Riverworld.

But why have humans been given another chance at life, and who is behind it all? That's what Sir Richard Francis Burton and Sam Clemens set out to discover in two earlier novels, one by riding the "suicide express" (if you die on Riverworld, you're resurrected again at a random point along the river) and the other steaming on the greatest riverboat ever seen. Now Milton Firebrass, Clemens's former enemy and now his No. 1 lieutenant, is planning to use the dwindling iron supply on the Riverworld to create a great airship, which can fly to the North Polar Sea far more quickly than any boat can travel. There he hopes to learn the secret of the mysterious tower thought to house the beings who created this planet.

After 33 years on the river, Clemens and his crew are finally near the end of their journey, and only one obstacle remains: the evil Earthly king, John Lackland. John is waiting just upriver in the Rex Grandissimus, the first riverboat that Sam constructed and the one that John and his crew hijacked, and he's hell-bent on sinking Sam's boat. Complicating the battle is the fact that both ships likely contain agents of the Ethicals, the group of advanced beings who created Riverworld for reasons unknown. One or more of the Ethicals themselves may even be on board, as are various humans that the rebel Ethical, known as the Mysterious Stranger (but known to Clemens simply as X), enlisted in his cause, which may or may not lead to humanity's salvation.

Most began life anew, accepting without question the sustenance provided by their mysterious benefactors. But a rebellious handful burned to confront the unseen masters who controlled their fate, and these few launched an invasion that would ultimately yield the mind-boggling truth. Now Riverworld's omnipotent leaders have been confronted, and the renegades of Riverworld, led by the intrepid Sir Richard Francis Burton, control the fantastic mechanism that once ruled them. But the most awesome challenge lies ahead. For in the vast corridors and secret rooms of the tower stronghold, an unknown enemy watches and waits to usurp the usurpers.

In a nutshell, Riverworld is a vast, constructed planet where everyone who's ever lived on Earth since the dawn of history to 1983 is resurrected. This leads, naturally, to an infinite variety of character meetings and confrontations through amusing and ingenious historical "what-ifs". Like what? Like what if Shakespeare came face to face with Richard III.

This slender volume with its misleading title consists of the story "Jesus on Mars", and a fragment originally excised from the Riverworld series, here entitled "Riverworld War". "Riverworld War" is a thrilling, action-packed depiction of the battle between the two riverboats captained by Sam Clemens and King John, all of which will be familiar (at least in general) to readers of Farmer's excellent Riverworld series.

Short stories where Davy Crocket hoping to refight the Battle of the Alamo learns that there is no profit in revenge and an actor pretending to be Robin of the Hood helps Julies Vern to recover his Utopia from Al Capone. Saint mingles with Sinner, Viking could fight Mongols, Elvis could run for president if he wanted to and these are just a few of the possibilities on Riverworld.