Millenium takes time travel and ties knots in it which is the best part of the book. Varley has looked at the possibilities for paradox and worked out how "time" would deal with it. For example, once a time traveler has gone back to a particular time, that time is closed forever. You can imagine the plot complications that creates! There's a serious underpinning to this story. Man in the far future has so utterly poisoned the earth that life is no longer possible. Man, along with all other animals life, is rapidly going extinct. A handful of time travelers are trying to give the human race a second chance by flinging the dead into the far, far future. Much of the book is involved in the mechanics of going onboard an airplane about to crash with no survivors, kidnapping everybody and then substituting realistic mannequins. To be caught could create a paradox that would destroy the space/time continuum. It's a great idea. It's so good that the book survives the fact it has too few characters. There are two main characters, both interesting but not very likeable or involving. The ending feels a bit forced, as though the author didn't know what to do with his story so made an end with really cosmic significance. Still, it's a fun ride with interesting ideas and it's well setup for a sequel of what happens in that far far future when the airplane crash survivors wake up next to sailing ship survivors and Roman army survivors. Now that would be an interesting situation!