IN THE WAR at the End of the World, Key West is hit twice by dud missiles and so survives. In honor of this salvation, the town is renamed Twicetown, and 60 years later one of the missiles -- a beautiful object seemingly made by the United States itself -- has become no more than the site for important events, where couples court and speeches are given.

This is the period described by Denis Johnson in his second novel, Fiskadoro -- the period of Quarantine when the rest of the United States and perhaps of the world has been destroyed.

It should be probably said right off that the novel has no plot; it is all situation. But despite this, Fiskadoro is a marvelous book, beautifully written and constantly entertaining.

It begins with the meeting of Fiskadoro, a boy from the Army -- the old Key West military base where fishermen now live -- and Mr. A.T. Cheung, manager of the Miami Symphony Orchestra, an 18- piece group ("but some of them were only 'political' appointees") that meets in a deserted airplane hangar to play reggae or, as its leader says, "We must do the blues. We must do the blues, and we must do the Voodoo."

Fiskadoro has inherited a clarinet and wants Mr. Cheung to teach him how to play it. At first the boy seems to have no talent, but then after his past has been wiped from him in a grisly self- mutilation ceremony among the swamp-people, he becomes the player with the spirit because he is able to forget himself and turn himself into music.

Forgetting is what much of this novel is about. The pre-war past is hardly remembered or is turned into such a confusion of myth and rock 'n' roll that it can't be said to exist at all. Mr. Cheung broods incessantly about the past. He is one of the few people who can read and has memorized the Declaration of Independence and the Constitution, which he recites when he's depressed.

"He wanted to bring back the other age -- just to get a look at it, the great civilization of helicopters and speedboats and dance parties atop buildings five hundred meters tall -- but there was nothing he could do but let that epoch pass, as it already in fact had, and to sit here with his clarinet in his lap, smoking marijuana in a cool Meerschaum pipe until the sun fell and sadness overcame him."

The only person who knows the truth of the previous age is Mr. Cheung's 100-year-old grandmother, Marie Wright, the oldest person on earth. But she can no longer speak, and so Mr. Cheung can learn nothing from her. But we are often in her mind, and for Grandmother Wright the End of the World began in Saigon in 1974 when her father, a British importer, shot himself and her mother went mad.

BECAUSE he loses his memory and has no past, Fiskadoro is ready to welcome the new age. The novel is told from the viewpoint of someone looking back to this time of Quarantine after Fiskadoro has become a leader. We never know this person although his people are approaching the shore at the end of the book -- people seemingly a mixture of Cuban communists, Muslems and Rastafarians. "Thinking about the past contributes nothing to the present endeavor," says the narrator. "But we are human. Can we help it if sometimes we like to tell stories that want, as their holiest purpose, to excite us with pictures of danger and chaos?"

Denis Johnson's highly praised first novel Angels concerned a few bleak months in the life of a young man who hoped to improve his lot by robbing banks and ended up going to the gas chamber in Arizona's Florence Prison. One of the many things that is impressive about Fiskadoro is that it is as different from Angels as a cotton shirt is different from a rock. Both books are told in short sections, both exist under the shadow of popular music -- rock 'n' roll or reggae -- but beyond that there is no similarity except for the astonishing writing.

Johnson has the tremendous gift of being constantly believable. Some details are hard to credit, and one has the sense that he never even looked at a map of Key West much less visited the Keys. But none of that matters. One never doubts him. The book moves from fantastic detail to fantastic detail, and one is constantly charmed, amazed, swept away.

There are better constructed novels than Fiskadoro -- it doesn't seem so much built as splashed out upon the page -- but there are few that are more impressive or enjoyable or visionary, and with it Johnson firmly establishes his place as one of our very best contemporary writers. He is a wonderful storyteller, and if at times Fiskadoro seems a mixture of Samuel Beckett, Philip K. Dick and Road Warrior, that is only to its credit.