This problem is an underrated masterpiece; it only got a 1st Honorable Mention in an informal tourney.
The point is that the Ng8 must go hide somewhere so that Black can castle, and, amazingly, b8 is the only possible square. It's eerie to look at the wide-open board on Black's 4th move and realize that, even with all that time to move the N around, there is no other square to leave him that does not interfere with some other piece's movement later on.
Edit: Hope you solved it instead of digging up the solution from PDB!