I think Parfit presents his ethical views with amazing ingenuity and nearly-robotic clarity, but passionate determination too. In the second half of the book, he argues that we can unify the most plausible versions of three major ethical theories: Kantianism, Social Contract Theory, and Consequentialism. According to his "Triple Theory", we should follow the set of principles that everyone could rationally will to be accepted universally, because such acceptance would make things go best. (p. 413)
In the first half of the book, Parfit tenaciously advances his theory of Objectivism about reasons. This view says facts can give you reasons to act, desire and believe things, regardless of what you may want. Here are two examples: 1) If you can save a bleeding stranger's life just by calling an ambulance, then that fact gives you a reason to do it, even if you'd rather not. 2) Everyone has a decisive reason to avoid agony.
Subjectivists must deny such intuitive claims. On their view, you have a reason to do something just in case you want to, or would want to after careful, informed deliberation. Besides having counterintuitive implications, Parfit argues, Subjectivism is groundless. If our desires are not fundamentally supported by some reason(s), then they are merely arbitrary. Arbitrary desires could not then give us reasons to do things. "So Subjective theories are built on sand," he concludes. (p. 91)
I think Parfit makes many important refinements of Kant's views on respect, consent, desert and the Golden Rule. To my surprise, he seems to show that it's not always wrong to use someone merely as a means to your own ends, and harm him without his consent! For example, suppose that an egoist, his baby daughter and a stranger find themselves trapped in some collapsing wreckage during an earthquake. The only way the egoist can save his daughter's life is by forcibly using the stranger's body in order to shield her from the falling wreckage, causing his leg to bruise. Pace Kant, it does not seem horribly wrong for the egoist to save his baby's life in that way. (p. 231)
Parfit's normative and metaethical views have certainly grown on me. I recommend both volumes of "On What Matters", as his arguments are characteristically inventive, lucid and comprehensive. See my review of Volume 2 if you're curious about that.