It has four parts. Justice cannot be done to the full arguments here; the following is merely a brief overview.
Part 1 argues that certain ethical theories are self-defeating. One such theory is ethical egoism, which Parfit claims fails due to the Prisoner's dilemma.
Ultimately, Parfit rejects "common sense morality" on similar grounds. Parfit himself does not explicitly endorse a particular view; rather, he shows what the problems of different theories are. His only positive endorsment is of "impersonal ethics" --impersonality being the common denominator of the different parts of the book.
Part 2 focuses on the relationship between rationality and time, dealing with questions such as: should we take into account our past desires?, should I do something I will regret later, even if it seems a good idea now?, and so on.
Part 3 argues for a reductive account of personal identity; rather than accepting the claim that our existence is a deep, significant fact about the world, Parfit's account of personal identity is like this:
At time 1, there is a person. At a later time 2, there is a person. These people seem to be the same person. Indeed, these people share memories and personality traits. But there are no further facts in the world that make them the same person.
Parfit's argument for this position relies on our intuitions regarding thought experiments such as teleportation, the fission and fusion of persons, gradual replacement of the matter in one's brain, gradual alteration of one's psychology, and so on.
Parfit's conclusion is similar to David Hume's view, and also to the view of the self in Buddhism, though it does not restrict itself to a mere reformulation of them. For besides being reductive, Parfit's view is also deflationary: in the end, "what matters" is not personal identity, but rather mental continuity and connectedness.
Part 4 deals with questions of our responsibility towards future generations. It raises questions about whether it can be wrong to create a life, whether environmental destruction violates the rights of future people, and so on.
One question Parfit raises is this: given that the course of history drastically affects what people are actually born (since it affects which potential parents actually meet and have children; and also, a difference in the time of conception will alter the genetic makeup of the child), do future persons have a right to complain about our actions, since they likely wouldn't exist if things had been different?
Another problem Parfit looks at is the Mere Addition Paradox, which supposedly shows that it is better to have a lot of people who are slightly happy, than a few people who are very happy. Parfit calls this view "repugnant", but says he has not yet found a solution.
Self-Defeating Theories
Rationality and Time
Personal Identity
Future Generations
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