After a car accident leaves Ada in a coma, Quinten is brought up by Max and Ada's mother Sophia, in a country castle which they share with an unusual assortment of tenants. Among them are a locksmith, an architect, and a philologist, who have just the skills to train Quinten for the task for which he has been brought into the world. For that he must find Onno, who has first become involved in politics and then become a kind of hermit, and the two of them must travel first to Rome and then to Jerusalem — to undo the covenant God made with man. The Discovery of Heaven is set within a framing story in which one angel reports to another on how events have been manipulated to this end, to bring Quinten into the world and then to lead him to his goal — with some extra work to dispose of pesky astronomers who discover too much about the universe.
The Discovery of Heaven is a novel of ideas on a large scale, sprawling across science, religion, architecture, politics and more. Though intellectual in its preoccupations, it is never didactic: ideas are integrated with the plot and characters and never allowed to get in the way of the story. The overall tone is also light-hearted, though it never lapses into the comic. And Mulisch never pushes any of the ideas too far — at least, none of the science made me wince, though I'm pretty sensitive to misuse of scientific ideas.
These strong points, however, are paralleled by some obvious criticisms that can be levelled at The Discovery of Heaven. It is shallow, never treating any of its subjects at length, or that seriously. The theology of the framing story may trouble some and, though I never found suspension of disbelief a problem, having angels intervene to manage the plot is too easy. And the character range is limited: the principal characters all have lives centred on ideas (Quinten is a typically unconvincing child prodigy), none of the female characters are at all substantial, and many of the strongest elements are autobiographical.
If you're after a large but lively novel of ideas, however, held together by an engaging story, then I highly recommend The Discovery of Heaven.