"If only I knew then what I know now." You've thought it. And that's what happens in this book. Jeff, a weary journalist in a bad marriage, has a heart attack at age 43 and wakes up in his old dorm room, himself now the 18-year-old college student he once was. He remembers his "future." New choices now, new options. What should he do?

Get rich, of course! The Kentucky Derby is in a few days and he happens to know who won. He's off and running and by age 43 he is a billionaire, a titan of Wall Street. Then he has another heart attack and he's 18 again and back on campus.

Since money is there for the taking, he can choose any lifestyle he wants. His new path is hedonism (a lot of sex in this chapter). Then, at age 43....

This might sound a "How I found wisdom" kind of book and there's some of that. But the plot really kicks in when he finds Pam, herself a "repeater." The way he discovers her existence is a wicked clever invention by the author. They fall intensely in love and team up. Their aim: try to bend history.

Pam and Jeff have their differences. He's a self-fulfillment kind of guy. She's a do-gooder with an agenda. They clash. But it's clear they are soul mates.

Can science explain what is happening to them? Should they go public? With each repeat, are they sending a different version of reality spinning off into the universe? Are there other repeaters and should they try and find them? Author Ken Grimwood explores all these issues.

A pattern emerges. Each repeat covers a shorter length of time. They always die on the same date, but each return comes later in their previous life. Each time they return they are, of course, separated again and have to find each other. Jeff returns earlier than Pam does, so he has to wait around. There's a marvelous scene with teenage Pam, stoned to the gills on pot, giggling with her friends in a bar booth when her moment suddenly comes. Across the room she spots Jeff, patiently waiting, a stranger instantly transformed into the love of her life.

An amusing running commentary is on historic and cultural events between 1963 and 1988, which is the initial span of the jump. Jeff and Pam have literally seen it all before as it happens over and over.

As the jump span shortens, each return finds them with more baggage of their previous lives -- family, kids, mortgages, etc. The last jump is incredibly gripping. The span? Nine minutes.

Grimwood pulls surprise after surprise out of his hat. Plot is his strong suit. His prose is OK. This story is a difficult one to tie up and the ending shows that. But he does send the reader away with a glow.

Replay is an extraordinary feat of imagination and outstanding entertainment.